I used some Sharpie on the Combiner Wars Shockwave I got yesterday. I don't know if it looks good yet (or even like Shockblast, aside from being slightly greener and bulky), but I think it looks better than the flat grey he was before. I'll probably give the purple on his collar another coat. I don't want to try to pop his head off the ball joint, especially since there's some definite tightening in his arms. I similarly don't really want to do his legs, as it lends some continuity to his abs. For the altmode I made his reticle purple on the front and his kneecap lasers are now purple too. I started with his feet, intending to silver up the grey parts, but the silver Sharpie doesn't really alter the color too much. Some purple-on-silver helps with the fins in gun mode.
Not to be rude, but why use sharpie for large areas like that? I think they work best for small details where you can't rub off the ink.
I agree that the big areas are kind of a hassle. But I don't have any paints and I don't really trust my abilities with a brush. I also like how the markers don't affect the texture/ detailing of the plastic.
You never know about painting until you try it. Get some small, cheap, simple figure like a G2 gobot/01 RID spychanger and try your hand at it. You're bound to be better than me at it.
Suggestion, use your sharpies or whatever paint pens you have. lay it on think and evenly coated, look nice. Then use Future floor polish (clear floor acrylic) to seal what you did. let dry 24 hours, voila. no rub! But personally, Id rather paint, and use markers on areas where no rub will occur....like eyes, inside gun barrels, deep grooves, etc. Even then I may still seal with Future polish. Just my 2 cents