Commodore Amiga Hardware Reference Manual page 142

If the bit combinations from attached sprites are as shown in table 4-7, then the colors
will be taken from the registers shown.
Table 4-7: Color Registers for Attached Sprites
Attached Sprites
Color
Value Register
0000
Not used
*
0001 17
0010 18
0011 19
0100 20
0101 21
0110 22
0111 23
1000 24
1001 25
1010 26
1011 27
1100 28
1101 29
1110 30
1111
31
*
Selects transparent mode.
Sprite Hardware 129