How do I paint a dark wall a lighter shade?

You will really be money ahead if you use a good primer first. I suggest Kilz of Zinsser. Get it in white and it will save you the work of applying several coats of light paint over the dark kgreen.

As with any paint project, you need to wash the walls first and let them dry thoroughly. When painting a lighter color over a darker color, it is best to have a separate stain and color blocking primer like Kilz. If you don't, the lighter color will have bleed through and turn out "muddy" looking and no where near the color you originally selected. Do yourself a favor and head over to a stand alone paint store for a consultation, in the end you will be happy you did.

https://www.glidden.com/inspiration/all-articles/steps-for-painting-over-dark-walls-with-a-light-co

https://homeguides.sfgate.com/paint-light-color-over-dark-color-35583.html

https://www.nolanpainting.com/2015/06/26/painting-over-dark-paint-colors/

I have to agree with above, washing the walls thoroughly is priority. A white primer stain blocker next. Followed by the color of your choice. There really are no shortcuts to this process.

Prime it first then paint it

Definitely need to prime the wall first. Going from darker to lighter paint color priming is a must. Buy a good quality paint such as Benjamin Moore. The paint job will last longer and be durable to scrub dirt/finger prints off. They also sell paints with primer in it so its one less step to do.

Yes Deb. That's exactly why I mentioned it to you because many are not aware of this. Why prime then paint when you can do it in one step rather than two. Ask or read the paint cans and look for primer already in the paint. So much easier. I never buy paint unless it has primer in it already. Work smarter not harder.

I don't know Judi what kind of paint brand you used but that seems like a lot of work. I didn't need specialized brushes/rollers to paint and I purchased Benjamin Moore which is a little more expensive than the others but I prefer its quality and durability. Benjamin Moore goes on much smoother. If the 1st coat is considered the primer coat and the 2nd is the actual paint coat than that seems to defeat the purpose of the primer in the paint. It all depends on how dark the wall is that you are painting to determine how many coats of paint you will need.