BBC BASIC Host Interface Physical Colour Specification
           ======================================================

Physical colour numbers are used as the short physical colour in the VDU 19
command, and used in the COLOUR and GCOL commands on platforms without a
palette. The colour number is a bitmap of the colour to select. When
implementing a VDU driver to interface with other platforms this mapping
must be adhered to.

===Colour bitmap===
    7   6   5   4   3   2   1   0
  +---+---+---+---+---+---+---+---+
  | L | D | E | F | I | B | G | R |
  +---+---+---+---+---+---+---+---+
    |   |   |   |   |   |   |   |
    |   |   |   |   |   |   |   +---Red
    |   |   |   |   |   |   +-------Green
    |   |   |   |   |   +-----------Blue
    |   |   |   |   +---------------Intense
    |   |   |   +-------------------Flash
    |   |   +-----------------------Extension
    |   +---------------------------Border
    +-------------------------------Layer 0=Foreground, 1=Background
  
  This gives:
   0 0 e f i b g r - &00+n - foreground colour
   0 1 e f i b g r - &40+n - extension
   1 0 e f i b g r - &80+n - background colour
   1 1 e f i b g r - &C0+n - border colour

Extensions are implementation specific. For examples, the ZX Spectrum uses
bit 5 and 6 to independently select Flash/Bright and RGB, and ANSI drivers
select underline and inverse.

The BBC and RISC OS uses b3=flash, whereas all other platforms use b4=flash.
For visual compatibility, programs should use 0-7 for non-flashing colours
and 24-31 for flashing colours. On the BBC/RISC OS these will be standard
intensity flashing colours, on other platforms these will be bright flashing
colours where supported.

Where individual control is needed in platform-independent code, the following
coding method should be used:

  os% set from OSBYTE 0
  IF os%<8 THEN flash%=8:bright%=16 ELSE flash%=16:bright%=8
  COLOUR n                 :REM select the specified colour
  COLOUR bright%+n         :REM select a bright colour
  COLOUR flash%+n          :REM select a flashing colour
  COLOUR flash%+bright%+n  :REM select a bright flashing colour