Freeform gradients. A new feature of Illustrator CC 2019. You can now add a new type of Illustrator gradient, freeform, to paths etc instead of just linear and radial. The freeform gives you a whole range of diffuse color features where the colors are blended from greens to reds and blues etc. You can add two different types of freeform using points and lines (you can use them together as well with the same path, they are not exclusive). Select a path and then goto the gradient panel and click the freeform button. Now select the first point and change the color by double clicking the point and changing the color or changing the color via the color panel or via the stop entry in the freeform panel. Click somewhere else in the path and add another point and change the color for that and you will now notice that you have a lovely flow of color from one point to the next - you may already have multiple points on the path but I am assuming that you started with one. You can now click elsewhere on the path and again change the color there.
How to create straight lines. The key thing is to use a combination of points and lines. Select a path and then select the gradient tool and then goto to the properties panel and select the freeform option. You will now see the current points or the default points (if this is a new path with no previously added points). Select any points you don't want and drag them off the path. Leave the points you want to use. Go to a point and set to 'points' in the properties panel (this breaks any link to any other line) and then convert to 'lines'. Now hold down the shift key and move up or down etc and click to add a new line point. To stop, press esc to release the 'straight line'. You can change the color of the stops as you add them or just click them individually after you have finished your straight line. You can create all kinds of colorful lines as well as a maze etc of straight lines as well as zigzags, etc. Note that you cannot cross a line, it will just not allow you to add a new point / stop.
Modify the spread of freeform. You can also control the spread from 0 to 100 either interactively or via the dropdown for the stop in the freeform panel. 0% gives the nicest flow of color but if you want a more harsh color look then 100% can be used. You can go back to each of the points and move them.
Opacity for each point. You can also change the opacity for each point so you can have transparency with your path in one particular area or with multiple points. Select the freeform gradient tool and then go to any of the points on the path and then go to main panel for the tool or go to the properties panel and then go to the opacity entry. The opacity entry is a dropdown with 0% and 10% etc settings all the way up to 100% (there is no entry field to set the opacity to 56.5% etc). There is no entry for the blending mode so any change to the blending mode for the path will affect the entire path. The transparency / opacity modifies only that part of the path influenced by the point / line. If there is no path below the freeform then the opacity will just show the artboard's white background. If there is a path below the freeform then changing the opacity will result in seeing more or less of the underlying path or paths. All kinds of wonderful blends can be created using the opacity.
Delete points. You can delete the points by clicking the little delete button next to the stop in the gradient panel. You can also delete the individual points by selecting the freeform point and then pressing the del key. As soon as you delete the point, you will see all the colors of the design change to reflect the removed point so the greens and blues etc will spread into other areas of the path.
Using lines. You can also use lines (as well as points) with the freeform gradient. The line is made up of points but the lines feature spreads the color differently to that generated by the individual (and separate points). To start using the lines feature, go to the lines option in the panel. As soon as you click the lines option, the tool will then add a 'rubber band' from any existing point and you can then drag that rubber band around and see the color change for the entire path once you click the path with a new point. You can convert the points into a line by clicking on the point with the 'lines' option selected in the panel and then going to the other points and clicking them and they will be added to the line. You can add multiple points to this line and as you add them you can see the color of the path change (depending on the position and opacity and spread etc) and you can add additional points to the line. You can leave the line open (by pressing esc key) or closed by returning to the first point. If you have used the curvature tool then you will be quickly familiar with the action of the line feature as it works in the same way. You can add multiple points to the line. You can also change the color after adding the line and you can shift the points as well as add new points to the line and remove them by using the delete stop feature (wastebin / garbage can next to the stop). All kinds of amazing colorful designs can be created using this feature with all kinds of paths.
Appearance panel / effects. You can also use the appearance panel to add new fills to the paths and then add stops to the new fill and then use the blending modes to combine the multiple freeform gradient fills. For example, create a path. Select the gradient tool. Go to the panel and select the freeform. Create the points and lines as required with the tool. Go to the appearance panel and then go to the right side menu and add a new fill. The initial fill will be a solid color (an alternative is to use the duplicate command in the same appearance menu) but as soon as you apply a freeform to it, the standard points will be available and you will then be able to edit them as before. Go to the opacity entry for the fill and then set to darken or overlay or lighten or difference. Change the color of the points / line as required. You can repeat this with multiple new fills and change the blending mode and opacity so you can see the underlying entries and colors and the result will be a super colorful blend. You cannot use the feature with strokes though. You can use the appearance panel and effects to create beautiful designs in seconds and they are always live. You can take any path and then fill that with the freeform gradient and then go to the effects and add blurs or distortions to the design and the the effects / colors can be combined in infinite ways. For example, you can create a very basic ellipse and then add a freeform gradient to that design (such as adding points and lines of greens and reds and white for a highlight) and then go to the effects menu and distort / transform and add a transform. Set the center point for a rotation to the middle right instead of using the center of the path. Go to angle / rotation field and set to 36 and then go to the copies and set to 9 and set the preview ON in the transform panel. You will then see the ellipse repeated to create a flower like design. You can then go to the original design and continue to edit the freeform gradient by selecting that tool. You can then change the colors from green to blue or move the position of the highlight or add additional points or remove points or perhaps use the curvature tool to modify the shape of the ellipse or path. You can then go to the effects menu and distort / transform and then use the transform again and add a new transformation to the design. Set the scaling point to the center of the 'paths' and set the preview to ON and then go to the scale fields and enter 130 or 120 etc and set the number of copies to 5 or 6 and now you should see the petals of the design spread out like a flower. On clicking apply to accept the transform, you can then rotate the original ellipse as well as edit the colors of the design (again via the free form). All kinds of beautiful petal designs can then be created and the artwork can then be exported via the CC library panel for use in tools such as Photoshop and After Effects
Continue edit. Once you have finished editing the gradient, you can always click the 'edit' button in the gradient panel and continue to modify the design. This can be done when you are working with any tool. You don't need to click the edit button if you select the gradient tool, as the freeform will be instantly displayed. You will then see all the points and lines associated with the design.
Save to styles. You can save the new colors to the styles panel via the new command in that panel so you can then apply the effect to other paths very quickly. You can also combine the new colorful designs with other paths as well as adding effects and more to the artwork. The feature also works well with the new global editing feature. Sadly, you can use the gradient on the stroke with the freeform and you cannot create a new swatch in CC 2019. You can also save the artwork to the CC library so the work can be re-used multiple times.