IBM RT Series Hardware Reference Manual page 157

Color Plane
Register
BitS Bit4
0 0
0 1
1 0
1 1
Attribute Controller. The Color Plane Enable
register controls the multiplexer for the video
wiring. The following table illustrates the
combinations available and the color output
wiring.
Input Status
Register One
BitS Bit4
Red Blue
Secondary Blue Green
Secondary Red Secondary Green
Not Used Not Used
IBM Enhanced Graphics Adapter 17