I'm interested in making a jigsaw tile swap game using only 2 colors for the puzzles, which I think would be a really neat and interesting constraint. However, keeping the UI to black and white would significantly hurt usability.  is that within the rules? 

from what i know, not 100% sure, there can only be two colors on screen at a time

my understanding is that you can use any color (not just black and white) as long as only two are on screen

so yeah, the ui has to be the same color as everything else as far as i know

In the rules they only mention using two colors, they never specify shades. I just really don't want to be disqualified 

Shades are different colors. This jam is about 1-bit, 0 or 1, on or off, black or white , red or green, ... .You can't process more shades with only one bit.