[pgf-tikz] what's the basic layer equivalent of \pgfsys at color@rgb

[Tamas Papp]

Hi,

If I have a color specified as RGB, with a floating point number for
each channel between 0 and 1, what's the exposed API to set this with
the basic layer? (as stroke, fill, or both).

I am looking for something that calls \pgfsys at color@rgb without
involving xcolor much, for efficiency.

best,

Tamas