I have tried paint on FRN handles and found that it can look good in the areas that are not in contact with your hands as it wears off rapidly. The best results were obtained with thinned out model paint that was allowed to flow into the "valley" areas on the FRN scale. I used a sharp pin to guide the paint into the areas where I wanted it to flow. I would experiment with a cheap throwaway knife first to establish your technique. Of course it is important to clean and prep the area with alcohol or similar non-destructive solvent to remove any oil or grease.
As far as G10 goes, I don't think the end results look very attractive. G10 is relatively coarse and shiny spray paint does not look good in my opinion. I have seen people spray G10 with flat-finish safety orange for safety/visibility reasons. The results work well for the task at hand but it does not look that good. Above all you need to consider how the handle will look after it has been handled and dropped. Hope this helps.
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