It's red, blue, purple and green, no one can reach it, not even the queen. What is it?

“A sunset”

A sunset can display a range of colors similar to those mentioned in the riddle, but it is not typically described in the same way as a rainbow, and it can be observed or reached.

“The sky”

While the sky can appear in various colors, it is not something that can be specifically characterized by the colors red, blue, purple, and green simultaneously, nor is it something that cannot be reached.

“A fruit salad”

A fruit salad can contain items of various colors, but it is a tangible object that can be reached and does not represent the concept of something unreachable like a rainbow.