The version of MetaPost that comes with MiKTeX doesn't
process colors in btex...etex. Dvitomp ignores them
when generating the corresponding mpx file.
There is a patch dvitomp.ch that enables dvitomp to
handle color specials which is included in
teTeX's texmf/web2c/ChangeLog reads:
2004-03-24 Olaf Weber <email@example.com.
* dvitomp.ch: Support for color specials, like those
generated by the colordvi package. From Eddie Kohler
A discussion on that problem can be found on
(with relevant postings by Hartmut Henkel).
Is it possible to include that patch in MiKTeX?