Thats because almost any graphics card is capable of running photoshop and editing video. It doesn't take a mega £300 card to run these.
In answer to your question, as far as I know the more expensive cards will not allow any more colours that say a £50 card, what it does allow is more of the game details to be run and at a higher resolution. For video editing, unless its really serious stuff a basic card will do as well, same for photos. I would say for photos its more important to have a good screen.