I understand that the Dulux paint colour codes are based on
the NCS colour system. I'd like to convert between this
and the RGB/CMY systems, so as to be able, for example, to
identify the Dulux colour most closely matching a given 3-byte
hex code (e.g. in CMY, primary magenta as 00FF00).

I have been unable to find the mathematical specification of
the NCS.

I have read the material at the NCS website and would be
grateful if someone could tell me whether

a) the definitions of the elementary hues R, Y, etc. in NCS
are the same as in CMY and RGB?

b) the gradation between two neighbouring elementary
colours, e.g. R and Y, is linear (in CMY terms)?

c) a colour such as S 2030-Y90R can be made by
subtractively mixing
- 20% black (255/255/255 in CMY)
- 30% of the hue defined by Y90R
- 50% white (0/0/0 in CMY)?

If the answer to each of these questions is yes, conversion is
simple, but I suspect there is more to it than this. I know
that some NCS codes do not exist, i.e. some spaces in
triangles in the NCS colour atlas for given hues are left
blank. I am not sure whether this is because the colours are
imaginary or just because the required pigments do not exist.

If there *is* more to it than this, could someone post
sufficient information to allow conversion between NCS and CMY
(and therefore RGB), given standard illumination conditions?
If the answer to c) above is 'yes', all I need is a formula
for converting between the two codes for a given hue.

Obviously a formula for converting between NCS and any colour
system from which I can get to RGB will be just as good!

Failing this, does anyone know the RGB codes for the
elementary colours (red, green, blue, yellow) used in NCS?
Or the NCS codes for the primary colours in RGB or CMY?
I have looked on loads of colour conversion sites, but
haven't been able to find even this information, anywhere!

Many thanks in advance for help with this.

Yours sincerely,

Chris