In addition to what Overread wrote, water will pretty much dilute Contrast paints while they are still wet - if you ****** something up, just get a wet brush and wipe as much away as you can. Wraithbone and Grey Seer in pots also are good for covering smaller mistakes.
As for the model you're not happy with, you can try to respray with the Contrast color. It goes on pretty fine, so if you aren't heavy handed, you might be able to get away without obscuring detail.
If you need to strip, if you're in the US, I recommend Super Clean - it's in a purple jug and you can get it at Walmart or auto stores. Put your model(s) into a (preferably) sealable glass jar or other container and add enough Super Clean to cover the model. Let sit for a few days and you should be able to lightly scrub the paint off with a toothbrush.