I don't think there's a rule for which colour to use other than "pale". I pretty much always do it with a certain type of image as it gives it a better grounding so it's not as likely to look as if it's just floating around.
With Copics I'd typically chose something like a pale blue or blue-green or a light gray (either cool or neutral, depending on the colours in the image).
With Prismas I usually colour with a mid to pale tone (again, blue or gray) close up to the image and then blend outwards to fade it into the background.
HTH!
|