I have the feeling I've already asked this but cannot find the email thread.
For canvas-draw colors are longints. There are convenience functions provided by canvas-draw according to the docs at http://webserver2.tecgraf.puc-rio.br/cd/ but they seem not to be accessible from the egg.
This snippet seems to work:
(define (vg:rgb->number r g b #!key (a 0))
(u32vector-ref (blob->u32vector (u8vector->blob (list->u8vector (list a r g b)))) 0))
But it feels rather clumsy and non-trivial for a beginner. Is there a better or at least simpler way?