Koleston mixtones arent going to last any longer than a true semi, especially as you're then opening the cuticle more by using an oxidative colour.
What is the name (not just brand because some of the colours work better than others) of the pink and purple? Were they pastel tones? Because these dont last very long anyway, and no matter what you use wont last more than a few washes.
Also, check the condition of your hair. If it is very porous then the ends will fade very quickly. I get it on a bleached section in my hair, the ends are always in worse condition than the roots and so cant hold the semi colour as well.
If you arent already, switch to a sulfate free shampoo, washing with cool water as that will minimise fade drastically, and make sure to close the cuticle after.
A good thing to do is to mix up some of your chosen colour with conditioner and use this when washing hair as it tops the colour up ever so slightly. Also, if you did use pastel colours, try instead mixing them up by buying the strong version, e.g. hot pink and vivid purple and mixing them with conditioner. I find that usually works better than pre mixed pastel colours because you can control the intensity of the colour, plus you can always mix more up so the pot goes further.
