my advice would be to aim for several shades lighter than you think.... purple can look very dark on the table so keeping it light will be your best bet, I'd suggest a pastel purple as a base colour then you can either find a nice purple wash or depending on whether you want a cool purple or warm purple, maybe try a blue or red wash respectively, the army painter red wash has quite a bit of purple in it so might work well
I'd also suggest a bright base scheme to help the figures stand out, light grey ash world, snow base, pale desert would all probably help them stand out and avoid looking muddy
hope this is of use, there are far better painters on here than me who may have contradictory advice of course
good luck and make sure you share some pics, your armies always look nice, you do yourself a disservice! at this scale it's more about consistency and bold colours and you've achieved this with your admech and orks which look great!