Hi, i found the following bug (likely) some time ago:

When \textext (or any other text generating thing like btex etex etc.) is
present inside \*MPinclusions and \*MPinclusions are followed by \*MPcode
ConTeXt returns error:
Equation cannot be performed (color=vacuous).

Minimal example would be:

\startMPinclusions
picture p;
p := image(
draw textext("Foo");
);
\stopMPinclusions
\startMPcode
\stopMPcode

tested on http://live.contextgarden.net/