The Parker parts catalog at hand lists the following colors:
Black, Midnight Blue, Navy Grey, Cocoa, Forest Green, Burgundy, Teal Blue
Reg. "51" A-M and Standard. The barrel is part number 586-004-2 (Plastic) - specify color
The same colors are listed for part number 596-004-1 (Plastic) Demi "51" A-M and Standard.
For fun:
Super "21", 41 and Debutante
Black, Red, Charcoal, Coral, Turquoise, Light Green, Green, Dark Blue, Light Blue (when ordering specify Super "21", 41, or Debutante colors)
The catalog I have is not complete with many updates missing and numerous annotations indicating changes that are not documented in Parker literature I own.
I would be interested in the references given as to the Blood Red story. I have heard various versions of this but they are always a bit different.
I am aware of numerous documented and properly marked XM pens in colors that didn't make it into production and early (1940ish) colors that were apparently dropped or done in very limited runs. There is also considerable color variation in the early Vac pens.