Interesting approach.

What I do is create my colour palette made up of 16x16px boxes off the page.
Each has a black line width of 0px to show alignment better.
I then simply right-click on a pallete box and drag-click into position multiple times to build up that colour.

I have a datum box at an exact 16px multiple X- & Y-coord and simply use Snap to Objects.

A sort of Painting by Blocks.

if I am doing a trace of a cartoon, I place that on a top locked layer and with Screen Bleach onto a black page.
When you drop your boxes in place, you can see the coverage through it more clearly.

Acorn