[OT] a good color distance function

[Justin Johansson]

Can anyone please suggest a decent algorithm for
determining a "color distance" metric between
two colors?  The requirement is to come up with
a good algorithm for contrasting a computed color
from another color.

Should this algorithm (function) be dependent upon
whether the color model be additive (aka RGB) or
subtractive (aka CMYK) or other?

Also should a decent "color distance" function take
in to account the possibility of color blindness
disability amongst the potential audience?

Somehow me thinks that (in say an RGB model) a
Pythagorean square-root of the the sum of the squares
of the respective RGB axis deltas is, whilst
mathematically pleasing, not best practice for
contemporary ideals/political correctness for addressing
accessibility/visual impairment issues.

Thanks in advance for your input,

Justin Johansson