Paint
Make a picture for Fluxer. Click the cells, or turn a photo into pixels. The preview is the same KaTeX a chat message would draw, and the counters stop at Fluxer's limits.
Choose a photo. The converter builds a palette, then shrinks the grid until the KaTeX fits in a Fluxer message. Edit on the canvas copies those cells so you can change them by hand.
No photo yet. Zero colors means the converter picks as many as will fit.