So, I use gimp and have been watching tutorials on how to complete a color change, Yet nothing seems to be working. Do any of you know how to help?

I use PS, but I used to use GIMP so I can try to help. Personally, I like to start by greyscaling the horse (completely desaturated it, adjust brightness and contrast to liking). If it's a bay or has visibly darker areas on the body, I'll use the lasso tool with 5-10 pt feathering and use brightness/contrast to match it to the rest of the body. I then use color balance, saturation, and brightness/contrast to get the color I want out of it. I then select areas with the feathered lasso tool to add darker and lighter areas to add some realism back into the new color.