PitBull is using Hexadecimal color values which cannot be used as VertexColor's. You can go to
WoWWiki(I know everyone uses WoWPedia but I haven't updated my bookmarks yet.) for the hexadecimal and RGB values of each class.
Ex.
Mage color hexadecimal = #69CCF0
Mage color RGB (in this case) = 0.41, 0.80, 0.94
EDIT: If you're looking for a hexadecimal to RGB color converter go
here. The only problem is it gives you the full value (255) SetVertexColor needs a value between 0 and 1, so in order to obtain this just take the number it gives you and divide by 255.
If I put in "ff8844" into the converted I'd get back [R:255] [G:136] [B:68] then to put this into SetVertexColor just open up Calculator and do [255/255 = 1] [136/255 = 0.53] [68/255 = 0.27]
Then plug these in [self.bg:SetVertexColor(1, 0.53, 0.27, self.bg:GetAlpha())]