I used to do plenty of pixel art in MS Paint on Windows XP when I was young. Zoom in as far as it’ll go, and the pencil tool works pixel by pixel. Just need a steady hand. You could even use the eyedropper tool, some modifier key I’ve forgotten, and the eraser tool to replace one color in the image with another if you were editing existing sprites.

So many silly recolor "OC"s. So. Many.