Easy way to stay in lines:
Use the Lasso Tool.
Select the whole inside of the cup with the lasso tool and fill it in with the bucket tool. Make sure to lock the layer afterwards when staring with the coloring, to not paint outside or use a Clipping Mask Layer.
Color changes while painting despite using the same color:
If the underlying color becomes darker, even though you are painting over it with the same color, check your brush mode settings, if it´s set to multiply/darken or something similiar. Make sure to have it set to normal. Same applies to the layer mode, if you are using multiple layers -> make sure the layer above is set to normal and not something like multiply, darken etc.
Hope this helps :3
