From what I have tried, blue will leave a greenish tint, could be the food coloring brand. The green worked good on the less than stellar leather that I put it on. Might have been stained with oil or something. The Eco-Flow that I also put on it didn't want to work right either. After it dried, there was no rub off and with good techniques, will work great. On some of the bad areas, I tried thinning it with alcohol and got better penetration, still wasn't as good as well prepped, but enough to color it a little.
Something else I remember from the black Army boot days, was using sharpies on areas of the boot that was scuffed to plain leather. We could get "free" markers and tended to work better than the Kiwi brand leather dye.
Thank you for this idea. Better than spending $8-9 for a small project.