For context, here is the question on TeX.SE:

http://tex.stackexchange.com/questions/129297/define-colour-transparency-in-relation-to-existing-colour

I agree with Marco:

Are you sure it's a good idea to add another colour definition
> mechanism? Then we have
>
  \definecolor
  \defineglobalcolor
  \definenamedcolor
  \definespotcolor
  \definemultitonecolor
  \defineprocesscolor

That is a little confusing. I can understand a speed requirement, but
surely that can be taken into consideration beneath the definition?

\definecolor[A][r=1, g=0, b=0]
\definecolor[B][A][a=1, t=0.5]

That seems fairly reasonable. Also, why not embed colour spaces within the
command?

    \definecolor[A][colorspace=spot]
    \definecolor[A][colorspace=multitone]
    \definecolor[A][colorspace=pantone]

One command to define a colour, rather than several commands for specific
variations of defining colours.

Kind regards.