Contours and how to create them using Photoshop gradients tutorial
The first thing to do is to have a shape laye and that is created by using the shape option with the shape tools such as ellipse and rectangle etc. Once you have the shape, you can then add an gradient / outer glow style via the layer menu and layer styles and follow the various steps below.
Once you are on the layer styles panel and the outer glow, you will see a gradient dropdown, click that to display the gradient editor and then go to the bar in the center of the panel and click the bottom of the gradient line to add a stop, change the color via the color setting on that panel. Click in different positions along the gradient line to add more stops to the line. If you want a more even contour with the gradient then go along the line perhaps clicking at 0% and 10% and 20% etc and set the color from green to black and green to black and so on alternating the colors. You can make any number of contour variations by simply setting the positions to 0% 5% 10% 20% 50% etc as well as changing the color alternating, perhaps use two blacks to one color combinations etc (up to you). Once you have that gradient you can then either click OK to add the result to the outer glow panel as well as click the new button to add it to the gradients list. You now have your contour start point for the gradient and the selected shape.
It should be noted that you don't have to perform the above in the outer glow panel, the generation of the contour gradient can be done while selecting the gradient overlay, gradient fill etc as well and then added to the list for future use. You may like to create a large selection of similar presets and save them as a new GRD file for future use. It is a pity that there are no additional controls to modify the color of the preset as well as saturation etc for the controls (you could do this in a variety of plugins such as the KPT ones). You can find many contour themed gradients via the 32,000 gradients collection from this site
You can create a contour by using a gradient style preset made of multiple gradients stops (as described above) and add an outer glow style to any selected shape etc via layer effects / styles dialog in the layers menu. Change the color of the outer glow from solid color to the gradient. You can change the outer glow style technique to precise and set the opacity and blend mode (such as normal) and spread and size as required. In the example below the blend mode has been set to normal for the contour gradient.
You can stop at this point if you want a colourful contour for the shape but if you want a black and white contour or perhaps different colors, rasterize the layer effect via the layer menu rasterize layer effects command. You can then go to the image menu and adjustments and threshold to reduce the image to black and white contours. Below is a single ellipse with a single outer glow and rasterized and then threshold adjustment added
Below you have the original layer with the glow all rasterized and remaining as a layer and then a new outer glow is added to the artwork using the same preset. It is a pity that the outer glow as well as all the other features in the layer menu cannot have a multiple / copies / repeat feature (many apps do but PS for some reason does not). Note that the circles are a little cruder. This is not so notable if you use a more unusual vector path than a basic ellipse
In the example below, a cross has been added as a layer (a combination of two rectangles and merged) and the layer style has been rasterized via the layer menu and then another glow with the same preset has been added and then flattened and then the threshold adjustment has been added. You do not need to flatten the artwork, as you could just add an adjustment layer. You can also turn the layer into a smart object and then add a new smart filter / adjustment 'threshold' and the threshold will then only be added to the object. Set the adjustment setting as required to have more or less of the lines included
You don't have to use the threshold, you can leave them in color
You can add multiple vectors with multiple glows and the glows can be made up of multiple color stops. You can turn the artwork into a smart object via the layer menu and then add a hue / saturation etc adjustment and you can rapidly create a suoer colorful background by creating multiple copies of the object with different hue and saturation settings (or any of the other colorful adjustments such as LUTs)
You can find many tutorials on the site such as how to use the Adobe generator feature in Photoshop and how to use the Photoshop gradients with transparency tutorial and how to use the Photoshop gradients with text / type and how to use the new Photoshop search feature in CC 2017