The difference between male and female cardinals

As you can see, The male cardinal is much more bright and pretty, whereas the female has sort of a grayish white color. This is because the females need to stay camouflaged to protect the eggs while the males need to be prettier to impress the females and get a mate.

This goes for every bird btw I just thought I would use Cardinals as an example

Not necessarily.
Some birds are both the same color, and some (like the Red-necked Phalarope ) have the color roles reversed.
But I’m general this is a good rule of thumb

I didn't really mean like EVERY SINGLE bird just most of them

I know