She's a tortie, tortie cats are different from brindle coloured dogs.
The tortie colouring happens because the ginger or orange tabby colour is linked to a sex chromosome. So basically when the colour of the cat is being expressed if the X chromosome has the "be a ginger tabby" instruction of it and the cat is male i.e it only has the one X chromosome, he'll be a ginger tabby.
For a female to be a ginger tabby both her X chromosomes need to have the "be a ginger tabby" instructions.
In each cell only one of a paired cromosome is active. I.e in a female cell you've got two X chromosomes but only one of them is active and it's random which one is, so in Pixie in her cells where the X chromosome that has the "be a ginger tabby!" instruction is active you get the orange hairs and else where you get the colour that she'd be otherwise.
So yeah it's a completely random mix and can only happen in female cats (or in rare male cats that have two X chromosomes, they're infertile then and that's a genetic abormality).
I know I went really technical there.. the simple answer is just, she's a tortie