Adding to custom color:

When you select an empty slot for the custom color it will overwrite the selected color with white. 

If the custom color is already pure white it should not overwrite a selected color (pure white is always available and would not be stored as a custom color as it would be redundant (i.e. it already exists in the pallet of all programs). 

Also there should be some selection outline surrounding the custom color box so you know which one has been selected, currently there is no visual feedback to the user on which one they are currently on so they could accidentally overwrite a color by hitting Add to Custom Colors.