<phrase> attribute

Hello,

I just came accross the following on a test file:

<phrase role="red">...

I'm trying to figure out what are the valid colours that might go in the 
role attribute. But these don't seem to be defined anywhere. I can't 
find this on the DTD or RelaxNG at all. I've been searching for an hour.

Could someone tell me where I can get a list of colours that the role 
attribute accepts?

Cheers,
Daniel.
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