The Pallet does give you access to all the HSV values. The problem is that Game Maker doesn't colorize sprites, it blends. So you're always stuck with the color you started with in the sprite, and that color blends with whatever color you select from the Pallet.
If you start with a sprite that is mostly white, then you are able to access many more colors from it. But darker sprites are difficult to work with. Below is an example of light-blue.