The CRL's specifically say that no modern era armor is acceptable for Crusader and Neo Crusader builds. Now, you could buy a modern helmet and mod it so heavily that it no longer resemebled a Fett helmet but, the ammount of work and cost involved in that would probably make it easier to just scratchbuild one. If you look through these threads, you will see a couple of scratchbuilds, also Getal Gra;tua did a great scratchbuild on his Early Era Crusader. If you look at Larc's helmer in the modern era Mando's it could be a KOTOR era easily. He built it from a hockey mask and some sintra.
As for colors, any referenced can be used. Any similar to can be used. It it's totally out of universe or out of era I would avoind it.
Again, as stated in the SWTOR thread, nothing about SWTOR will be considered canon or effect our CRL"s until LFL has created a canon stryline for it, if they do. At which time we will reevaluate.
Additionally, as Slade has mentioned in teh SWTOR thread, SWTOR is set about 300 years after the conclusion of the major events from KOTOR. So, the crusader movement has been dead for 400-500 years by that time and likely the Neo Crusader movemet is dead as well.
The presence of the suspected Mandalorian in modern era armor, is in my opinion only, an Easter egg lik ethe same was done in the KOTOR comics. An Easter Egg of a dead Mandalorian armor very similar to Boba Fett's is seen on the battlefield. Again, that was done as an Easter Egg by the artists not as literal canon.