NextPort:$FF3B
From SpecNext Wiki
0xFF3B
ULA+ Data
(R)
if palette group is selected, the palette value stored at the current ula+ palette index is returned if the mode group is selected, bit 0 = 1 to indicate ULA+ is enabled
(W)
if the palette group is selected, a palette value is written to the current ula+ palette index if the mode group is selected, bit 0 = 1 to enable ULA+, 0 to disable ULA+
For ULA+ i/o ports, all colours are read and written in 8-bit format GGGRRRBB. The 9th blue bit is automatically generated as the logical OR of the two other blue bits.