Re: [Comsol 3.5a]how can i colour the subdomain?

I don't know well about the 3.5a, but I think it will be the same thing with 3.4.
In 3.4, you should open "Options-Preferences..." Then in the Tab "Visualization" check "Face" under "Render".
Then in the subdomain setting, you can choose a color manually for each group.

Attention please, you can only choose color when you are in the "Groups" tab of the subdomain settings.