Ohhh I used to be a professional framer, so I 
might be able to help you there. I would say go 
with dark purple/grape color (one that matches 
his clothes.) It makes for a more stunning 
effect.... (i hope you dont mind, but I edited 
your pic with photoshop to show you an example.)
If you did the picture with a red, it would not 
contrast as nicely. If you are willing to spend 
a little extra money, you might wanna consider 
getting a gold matting underlay for your cel 
(for a double matted look.) It might jazz up 
your cel a little. .........But otherwise I 
think the purple would look best. (Also, 
remember, you should always spend a little more 
and get acid free matboard if you can....(normal 
matboard can yellow your image after some time, 
especially if kept near sunlight....)