The Amiga has a special mode called Extra Half Brite or EHB mode which
doubles the maximum number of colors that can be displayed at one time. To
use EHB mode, you must set up six bitplanes.  Then set  bpu =6 (bits 12,
13 and 14) in the  bplcon0  register. set  homod =0 (bit 11) and  dblpf =0
(bit 10) in  bplcon0 .  in this mode, the information in bitplane 6
controls an intensity reduction in the other 5 bitplanes. The color
register output selected by the first five bitplanes is shifted to
half-intensity by the sixth bitplane. This allows 64 colors to be
displayed at one time instead of the usual 32.

   ECS playfield registers.
   -----------------------
   For  information  concerning the playfield hardware and the enhanced
   Chip Set, see Appendix C.