How to mix / combine colors with Photoshop gradients and color channels tutorial
You can use the Photoshop gradients in standard RGB but you can also use them in different channels such as the red channel and green channel and blue channel.
Go to the Photoshop channels panel (window menu) and select one of the channels (such as red channel - by just selecting the item in the panel and you will see the image changed to the selected channel if red or green or blue) and then apply the gradient; go to another (such as green) and apply the same or perhaps a different color effect or apply the same in a different manner or perhaps using a different blending mode; and repeat depending on the number of channels.
You can use them in all of the channels but you can just use them in the red etc and ignore the rest.
You can also mix the separate red channel and green etc with effects so you can add into the mix camera raw filter as well as blurs etc and you can also add in adjustments as well such as invert, posterize, equalize and threshold adjustments as well as adjustment layers in Photoshop to create all kinds of color effects and blends with the red / green etc
Also, many color effects can be added to the individual separate channels and gradients to create even more unusual color effects and mixes of gradients
You can use them in different channels in 8bits but you can also do the same in 32bits as well as 16bit though there are a number of blending modes lost when you use 32bit as well as filters. You can add them to the red and then add a different one to the green (you can ignore the blue or red etc as required). You can then also use different filters in the red / green etc with the added color. You can also add them and use the edit menu and fade command and set the opacity to 50% etc as well as use with different blending modes there also.
You can do all the same as the RGB with the CMYK. You can select the cyan and then the magenta and perhaps not use the yellow at all and perhaps ignore the black. You can also use the magenta and ignore the cyan. You can use the same GRD effect or perhaps a different one or perhaps use a different mode such as linear or angle or perhaps use different origin points for the tool. You can also combine the results of different cyan and magenta etc with effects as well as perhaps different effects in either.
You can also just split the channels via the same named panel by going to the right side menu. Why do this ??? well if you have add the color effects and you want to add an effect or two and suddenly find that effect is not available in the red channel or green channel then the quickest way around that is just to use the split. Once you split the file, you get a new file with the red and a new one with the green and a new one with the blue and then you can merge them all back together again or if you wish, merge them all back but with red using the green file and the green using the blue and the blue using the red, it is all up to you - though that is not totally true as the merge is quite fussy in that it allows you to only use them once only. When you split them into files, you get the artwork in grayscale so if you want to add an effect such as stylize oil paint tool then you will have to convert the artwork into RGB and then add the effect.
One trouble with the split is that it is very much trial and error as the artwork is now spread over three or four different images and where the middle point etc is purely by eye though to make it easier you could could use a grid of guide lines via the new guide layout and use the same guide layout in each. If you convert one of the single channels into a RGB file you can then split that as well and repeat the same approach multiple times before merging the whole lot together.
Sadly, you cannot bring another image into the mix though, of course, you could simply bring in another image into one of the channels and then merge them. Often there is a work around for some odd limitations (I guess it may be a size issue but if the image is the same size as the original, surely it would not matter). If you have converted a grayscale image to an RGB, you will have to also convert it back to grayscale before you can use it in the merge as well. You can also modify the end result even more with tools such as color adjustments so as color lookup
You can also use the gradients by taking the result of individual red and green and blue (if RGB) or cyan, magenta etc (if CMYK) and using them as a layer. Go to the panel and select one of the channels and then use a selection (select all) and copy that and paste as a new layer. Go to the green channel and repeat and paste that as a new layer and then blue etc as a new layer so now all are individual layers and also you still have the original which can be ignore if the blending modes of the new layers make the layer invisible (you can also just set the layer to disable or delete it). You can set the blending modes for the layers to difference / darken etc and you will then see the underlying layer source and you can really use that to intensify all the colors in the original. As the layers can also save them to the CC library but to do this you must disable or remove the top most layers and then drag the channel layer to the CC library and store the result for future use.
Changing Image Modes with Photoshop Gradients Tutorial as well as others such as how to create amazing frames / borders with Photoshop gradients and how to combine multiple gradients in Photoshop using blending modes tutorial and how to use custom shapes to create patterns
You may find the Photoshop gradients of interest and use with PS and PS elements with the red, green etc
Some more examples of mixes of gradients across channels in Photoshop