Can’t paint lighter colors on darker colors on the same layer

It sounds like you might have accidentally turned on the "Lock Transparency" option for your layer. This would prevent you from painting any colors on the layer that are not already present in the transparent areas of the layer.

To fix this, you can check if the "Lock Transparency" option is turned on for the layer that you are having trouble painting on. You can do this by selecting the layer and looking for a small "checkerboard" icon at the top of the Layers panel. If the icon has a small lock symbol on it, then the "Lock Transparency" option is turned on.

To turn off "Lock Transparency" and allow painting with any color, simply click on the "checkerboard" icon with the lock symbol on it. This will toggle off the "Lock Transparency" option.

If you want to make sure that you don't accidentally turn on "Lock Transparency" in the future, you can also disable it from the Layers panel menu. Go to the Layers panel menu (the small icon in the top-right corner of the panel), and uncheck "Lock Transparency" in the dropdown menu.

I hope this helps!

Could you please post screenshots with the pertinent Panels (Toolbar, Layers, Options Bar, …) visible? 

I reckon Dave is right, it will be the brush blending mode. Try "normal".

I hope this helps
neil barstow, colourmanagement net - adobe forum volunteer - co-author: 'getting colour right'
google me "neil barstow colourmanagement" for lots of free articles on colour management

With the brush selected look in the options bar at the top of the screen. The brush will have a blending mode (it is labelled 'Mode:') Set it to Normal

Dave