What is the white cake with rainbow sprinkles inside it called?

There's no specific name for a white cake with rainbow sprinkles inside. You could call it a rainbow sprinkle cake, or a white cake with rainbow sprinkles.

Some people might call it a funfetti cake, which is a popular brand of white cake mix that comes with sprinkles. However, funfetti cake is often made with yellow cake mix, not white.

Ultimately, the best name for your cake depends on how you want to describe it!