Kram,
Not speaking for Nick, but in general, unless Colin was a member of the band's corporation, it really doesn't matter. I don't think he had any ownership. My full-on guess is, Porcupine Tree was probably just SW and for this revamp, they re-did it so it was SW/RB/GH -- thus their increased roles in the business.
As for Colin's royalties, for records he performed on, those are generally captured and sent to him by whatever group (ASCAP, BMI, etc.) handles his rights. I'm not sure he had any publishing (maybe he did)? If so, publishing (writing) royalties are handled in a similar manner.
I'm not an expert on PT's structure, so this is all in generalities.