At-the-money gamma rises sharply as expiry nears, while out-of-the-money gamma fades toward zero. Colour is the number that quantifies both effects, and it is what makes a quiet short-dated position turn dangerous overnight without any price movement.
Desks that carry short gamma into expiration watch colour to know when a position that is currently manageable becomes unmanageable. It is the calendar equivalent of speed-greek.
Example: short ten XYZ $50 calls with XYZ at $50 and four days left. Today the position's gamma implies a $180 delta shift per dollar. Two days later, with the price unchanged, the same position implies $310 per dollar. Colour did that, not the market.
Related: speed-greek, gamma-risk, time-decay-curve, second-order-greeks