You will be unable to Add photos to an Album while the +Add Photo button is RED.

There are several reasons why an +Add Photo button might be RED.

The reasons will vary depending on the Album type, and other factors.  

If you click on a RED +Add Photo button, SnapBraggin will provide an explanation. 

For a full list of reasons why the +Add Photo button might be RED, please read: I am not able to Add Photos to an Album. Why?