Pixels

Command-line per-pixel texture editor
Github

Pixels is designed for textures, specifically textures that are very similar to existing textures and just need repetitive tasks done to them: swap the red and green components to turn a ruby into a emerald, copy the green value to the blue component for a diamond, swap the green and red components to make an amethyst, and equalize the three values for quartz. Pixels can do all of these operations with one command, allowing you to focus on editing one file and watch the changes propogate to the rest.