From OHRRPGCE-Wiki

As has been said elsewhere, MS Paint, the paint program included with Windows, is not the best image editing program out there. Meaning it pretty much sucks. But some people are stubborn, and would rather figure out how to do their OHR graphics in the program they already use than figure out a new one, and in many cases that means MSPaint.

MSPaint's main shortcoming is its palette handling... specifically, it has none. Any and all images you save as 16-color bitmaps are remapped to exactly the same color palette, which consists of black, white, and two high-contrast shades each of gray, red, yellow, green, cyan, blue, and magenta.

In most situations, these aren't going to be the colors you want to use. But if you use any other colors and then save a 16-color image, who knows what the heck MSPaint will do to your image? If any of the colors you used get mapped to the same thing, you'll lose detail, and on a 20x20 sprite, detail isn't really something you can spare.

What you want to do, then, is use colors that you know ahead of time MSPaint will map into separate palette colors. Where can you find colors like that? Why, they're right there in front of you! I just pointed them out in the image above!

"But those colors are ugly!" Well, fear not. You still get to draw your sprites using whatever colors you want. As you go, keep track of how many colors you've used, and draw yourself a little 4x4 (or whatever shape works for you) palette to work from. Now, before you save in 16 colors, use the eraser right-click to replace each color you actually used on the sprite with a different one of the MSPaint 16 colors. Be sure you do this the same way for every sprite that's intended to use the same palette. (Also definitely keep a backup of the sprite.)

When you import the sprite into CUSTOM, for the first sprite to use a given palette, you'll have to go through and manually reassign all the colors to what you really wanted them to be. After that, other sprites you save this way will map into the existing palette correctly as long as you always pick the same background color.

All this only applies for versions of MSPaint from the last ten years, i.e. included with Windows 95 and up. If you're using anything older, it only lets you draw in 16 colors anyway, so there's no real point in worrying about them.

[edit] Alternatively

There's another method, which is slightly more complicated, but works anyhow. MSPaint will preserve a palette in an image that already has one (i.e. an existing 16 colour image).

So, if you want to put a little bit of effort, you can use a real drawing program to assign a palette to a blank picture, and save that. Then, use it as a template in MSPaint.

Or, heck, make the appropriate palette in the OHR, and then use the OHR Graphics Exporter to export a sprite with the palette.

Either way, you can then draw with the colours and not worry. Of course, if you draw with any others, MSPaint will try and map them to yours, so be careful!