You can always create your own custom color lookup tables. Just place a text file with the extension .clut in MRIcroGL's /Resources/lut folder (for MacOS users, right-click on the MRIcroGL and select "show package contents" to see these folders). Since Violet is red+blue, here is a simple violet color scheme that goes from black (RGB = 0,0,0) to violet (RGB = 255, 0, 255). Name this "Violet.clut" and place it in your out folder:

[FLT]
min=0
max=0
[INT]
numnodes=3
[BYT]
nodeintensity0=0
nodeintensity1=128
nodeintensity2=255
[RGBA255]
nodergba0=0|0|0|0
nodergba1=128|0|128|64
nodergba2=255|0|255|128