How do I stop colours from overlapping each other in Photoshop?

Hi i just started using photoshop now so I'm a bit new to it but when i colour in a region and then left my pen and colour it again the original colour gets darker and i want to find a way to just keep it at one tone.How do I stop colours from overlapping each other in Photoshop?
Make sure that the opacity is set to 100%

over the counter cream