Let's say I'm filling in a cube with light purple, pixel by pixel, but one pixel is dark purple. I've noticed that it only happens on a specific layer, but copy-pasting it to another layer will keep the problem. I'm totally new to photoshop so I apologize if I can't help with correct terminology.
Please to post screenshots (including all pertinent Panels).
Are Adjustment Layers involved?
Wow that was an easy fix. I had the mode on "difference". Now I know.