The lighter the square in a heat map, the earlier in the solve the correct letter was entered in that square.
No real pattern is visible in M&A's puzzle because it was challenging and had to be completed in bits and pieces.
Acme's puzzle, on the other hand, was a quintessential Monday puzzle that offered me no resistance. The colors indicate that I solved it top to bottom, starting in the NW and ending in the SW.
Masked and Anonymous 12 Days of Christmas puzzle
Acme's debut NYT puzzle, 6/12/2000