There are numerous online guides to budgerigar (budgie) mutations. Here are the four main categories (according to budgieplace.com): Colouration Mutations, Striping Pattern Mutations, Pied Mutations and Rare Mutations. Each category is comprised of numerous subcategories; the first under "Colouration Mutations" is "Base Colour":
"All budgies fall into one of two basic varieties. Either they have a yellow pigment base (dominant) or they lack a yellow pigment base and are therefore white-based (recessive). In general, the base color is visible in the mask feathers and between the black stripes of the head and wings. (The exception is the yellow-face variety.) Normally, the body feathers are structured to reflect blue. In yellow-based budgies the blue in the body feathers combines with the yellow base pigment, which results in a bright green, the most common variety. In white-based budgies there is no yellow base pigment, so the blue structure of the body feathers results in bright blue coloration."