washing my dirtbike plastics?

i have an 02 cr250 that i bought recently, it had brand new plastics, they are red and white, but i took it on a test ride in th wood for the 1st time and you know it gets dirty as usual, but my white plastics are like white and brownish color because of the dirt, ive been washing the white plastics over and over but it stays the same, any one has a certain product they use or a certain way to get back the white back on my plaqstics?