If my edible images are going on a colored cake, I will make sure my graphic is on a similar background as the cake. So in your case, I would have put my black logo on a red background in my graphics program. Then when you print and cut it out, the small bits of red would blend into the cake, not stand out. Hope that makes sense!
Oh, I HATE coloring fondant! I keep fondant in most of the base colors (white, black, red, blue, yellow), but I have been known to order a specialty color if I need a lot of it (like baby pink or tiffany blue). Otherwise I just mix it up.
Honestly, the baking of the cakes is probably the simplest, least time-consuming part of the cake making process. So if I was in your situation, it wouldn't change the price that much really at all. You still have to be paid to get there (you also should be compensated for all the other work you would miss) and to decorate the cake. And I wouldn't really even consider decorating someone else's cake. I know the cakes and icings I make like a dear old friend - I know...
I love modeling chocolate! There is a Craftsy class on it that I highly recommend for beginners (of the medium). I have had TERRIBLE luck with using edible markers on modeling chocolate when you are going to put the cake in the Refridgerator. It has run, dripped and looked terrible when you take it out and it gets condensation (an hour or so out). I will not use the pens again for this reason. Good luck!
I keep all my cakes in the fridge - fondant and buttercream. And they will sweat when I take them out. Just don't touch the cake and it should dry out in about an hour (maybe less). I haven't tried the new Wilton fondant and I didn't realize the old Wilton fondant didn't sweat.