transparent metafun colors turn nearly black

[Sanjoy Mahajan <sanjoy@mrao.cam.ac.uk>]

Just for (meta)fun, I tried a few experiments with transparent colors.
The example below gives a black circle instead of a red one.  Is it that
transparent colors aren't part of metafun -- which I can easily live
with since color printing is too expensive still, alas -- or have I
omitted an essential command?

==================== cut here ====================
\runMPgraphicstrue
\setupcolors[state=start]
\starttext
\startreusableMPgraphic{a}
% _inline_specials_ := true; % had hopes for this line, but no luck
% next line taken as an example from mp-spec.mp
fill fullcircle scaled 10cm withcolor transparent(8,.3,red);
\stopreusableMPgraphic

\placefigure[force,none]{}{\reuseMPgraphic{a}}
\stoptext
==================== cut here ====================

The metapost output (testfile-mpgraph.1) always contains the following
color setting, no matter what arguments I pass to transparent() :

 0.123 0.003 0.001 setrgbcolor

Which is close to black.

(This is all with ConTeXt 2006.08.08.  The live ConTeXt gives a square
saying "undefined".)

-Sanjoy

`A society of sheep must in time beget a government of wolves.'
   -- Bertrand de Jouvenal