Janine is right, directions white toner, and purple shampoos.. and basically anything intended to neutralise brassiness is usually purple,
In regards to the diluting, white conditioner makes them paler, but for example I diluted a pink and it came out just a lighter neon pink, so to make it softer I added a tiny bit of blue, so you can dilute colours with conditioner but they won't necessarily become 'pastel' immediately, you might have to fiddle a bit, although purple is usually fine (pastels just tend to be a bit cooler, so some colours need a bit of balancing)