Paints and bettas are two different universes. lol
If you want a blue/purple cambodian, try mixing red cambodian to blue. You should get some red cambodian, blue with red fins, and some blue cambodian (depending on the genetic back ground). If you don't get any blue cambo's in F1, cross it again to blue. You should get BC.
Even better, you can mix opaque to blue and should get mostly blue cambodian/opaque ..... I'm not sure what they call the color.
As stated above, a true purple is suppose to be impossible in the betta world. But who knows, maybe some day they will mutate into such color.