You could, but if you get too close to black on Paint, it automatically just calls it black. But, yes, using a really dark brown, for instance, instead of straight black is how I prefer to do it.

or use an artist black (a dark grayish color thats near black)

For future reference, if you use MSPaint: you're coloring a render and want to create a black horse, or one with black markings, but you have a problem- the lines are black. So what do you do?

Choose another color, say... green's a good color(you can change it later), and set it as your main/first color. Then, right click the color black from the color menu. So, your first color is that lovely green and your second, the back color, is black.

Now, choose the eraser tool. Normally you would click where ever you want to erase, and the tool will make white boxes appear all over your image. But not this time. Right click instead of clicking normally and drag your eraser over a black line. IT CHANGED GREEN! Yeah, it does that. You see, whatever your second color is, black, that's the color that's going to be replaced with your first color, green? Get it?

Now you can fill in your horse black, and change those nasty green lines to whatever other color you want them to be.

That's how you color a black horse, but keep those important lines. And if this doesn't make much sense, I have decided to do a tutorial about this, and other "Coloring Renders" topics. Just ask, and I'll see what I can do!