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.