The SATS boarding passes I've seen handed out in Changi (never for SQ though) are very basic, just a white pass with a strip of color up top, and follow the same color scheme as SQ's passes: Green for Economy, blue for Business. Probably red for First too, but unfortunately I can't vouch from personal experience.
Orange for Executive Economy (or Executive Business?) sounds like a reasonable guess to me, although I'm not sure why they'd need to distinguish it as there is no 'normal' Y or C on the A345s.