Additional pixels when drawing with low value color

When drawing a line with `Color444(0,1,0)` or `Color888(0,16,0)` then a shadow line appears below, with some blue and red. This is the lowest possible pixel value, apparently, below this value there are no pixels drawn at all.

Reproduce the issue by drawing a line with Color444(0,1,0) or Color888(0,16,0), then trace the library's pixel and line-drawing entry points to find where the shadow pixels are introduced. Done means the lowest nonzero green values draw only the requested line, with no unintended red or blue pixels.

Written by the indexing model from the issue text.