Train your eye to instantly judge whether two squares share the same color. The two-square companion to the square color drill: it builds the parity sense that powers bishop play and spotting which squares sit on the same diagonal, all without looking at the board.
Pro tip: Two squares share a color when their files and ranks have the same parity. a1 and c3 are both dark; a1 and b2 differ.
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Leaderboard
About Same Color
Judging two squares at once
Two square names appear and you answer whether they share a colour. It is the two-square companion to the single-square drill, and it is a genuinely different skill rather than the same one twice. Answering "is d5 light" is a lookup. Answering "are d5 and g2 the same colour" is a comparison, and comparison is what you actually do at the board when you are working out whether a bishop can ever touch a given square.
You can get there two ways. The slow way is to resolve each square then compare, which works but doubles the cost. The fast way is parity: two squares match when their file gap and rank gap are both even or both odd. a1 and c3 are two apart in each direction, so they match. a1 and b2 are one apart in each, so they also match, which surprises people who expect adjacent-diagonal squares to differ. Once this is instant you get a free filter on every diagonal question, since squares of different colours can never share a diagonal or be reached by the same bishop. Build the base reflex in square colours, then take the parity sense into the diagonals drill and the bishop maze.