According to https://learn.adafruit.com/32x16-32x32-rgb-led-matrix/wiring-the-16x32-matrix the recommended wiring for the 16x32 matrix on a MEGA appears to be:
R1 goes to digital 24
G1 goes to digital 25
B1 goes to digital 26
R2 goes to digital 27
G2 goes to digital 28
B2 goes to digital 29
CLK goes to digital 11
OE goes to digital 9
A goes to analog 0
B goes to analog 1
C goes to analog 2
LAT goes to analog 3
It looks like A0 and A1 are used by the Ethernet shield so using A2, A3, A4, and A5 instead makes sense.