I have a gallon of new paint but it is too dark. I would like to know how to lighten it.
The only way is too add some white paint. This could be a problem and will add to your expenses.
1- You will need to purchase some white paint of the same type and sheen. It could take a quart or more depending on just how light you want to go and how dark your paint is now.
2- This will be a truly custom color. If you ever need more you will need to remember the exact formula used. Most likely you won't be able to completely duplicate the color unless you use measuring cups and keep track of everything.
It is so much easier to just choose and buy a lighter color from a fan deck. Make sure to always test or sample a color before purchasing any volume.