What should I use to paint a purple wall in cream color?

Behr makes a paint that has primer in it. We used it on new drywallb & got great coverage w/ one coat. Changing the color from-) - I assume a dark one since you say purple rather than lavender - to light, you will not be able to avoid a second cost simply because of the marks your brush will leave as you trim. Still, buying one product rather than 2 is always less expensive & less tools to clean after. Behr will do it easily w/ 2 coats. Most others will require 2 & a touch up where you trim. Home Depot carries Behr. and Behr has great colors:)