graphicxtras.com > Photoshop shapes and image modes such as CMYK, RGB, tritone, duotone, grayscale etc tutorial
You can set PS into number of color modes such as RGB and CMYK and LAB etc and this is done via the image menu and mode and then select the required image mode as well as the color bits per channel such as the standard 8bit or 16bit etc for better color. You can use custom shapes in most of them
You can add the artwork as a layer or pixel in RGB / CMYK etc. Simply select the CSH tool in the toolbar (found normally beneath the rectangle tool but now with the new toolbar customization this tool can be anywhere in the toolbar) and just add. You can then add layer effects and colors to the artwork though the actual results will depend on the color mode. Such as below where an image was originally in RGB and then it was converted via the image menu into grayscale mode and then into duotone (never understood why there is not a direct conversion between the tool perhaps using the powerful black and white adjustment as an inbetween - of course, you can always convert the RGB to grayscale using the Black and white adjustment and then convert to grayscale image mode. Once in the duotone mode, the CSH tool was used and the result is a duotone vector layer and the gradient added as a background was originally a super colorful red and yellow artwork but turned into a blue gradient instead.
You can add the vector layers and pixels to CMYK as well. You can also select the individual channels of cyan and magenta etc and add the artworks to those as well as below where the baubles are added in different locations on the image. Any number of vectors can be added to the image and the color of the vector can be the same or different in the individual channels. You can do the same in 8bit CMYK as well as 16bit CMYK
You can also add the artworks to the image in RGB and 16bit and 32bit mode. You can add them as a layer as well as a pixel in 32bit mode (though not layers in the individual channels). You can also use a selection of effects to combine with the shapes such as the color lookup adjustment. When you convert back to standard RGB 8bit mode then HDR toning panel appears and then you can tweak the color conversion in numerous ways and create some very subtle color works as well as some truly extreme ones as well as blurry and smooth changes.
You can find the items used on this page in the Christmas custom shapes section, they can be used with PS and PS Elements