Here's a tutorial how to make a bit more realistic dapples in MS Paint. Dapple is one of the hardest horse coats to make and this technique needs some patience but for me it looks far better than random spots made by aibrush.
1. First of, observe how dapples look on real horse. There are irregular spots fading in some parts of a coat:
2.Start your image with base coat.
3. Keeping in mind what we observed in first step, start drawing freehand, irregular spots.
4. After some time of patient work, you have all dapples done. You can leave it now as it is but it still looks kinda weird.
5.Change some dapples to darker colours to have this fading markings effect real horse has.
6. Some additional final touches by big airbrush. Do not ooveruse it!
Lineart by Blackhorsebeth