I love the Gallop color enhancing shampoos. I use it on Cinny before his shows and it helps him look black. I used "Dark as Knight" last year, which can also be used on bays and it kept him pretty dark, but as mentioned it's mainly Paprika that does it.
If you are going to schooling shows, I wouldn't worry so much about getting positives in drug testing as they usually don't drug test at local and schooling shows. If you are planning on any breed shows, or USEF show's I'd be VERY careful with the Paprika. You need to stop feeding it a minimum of 2 weeks before the show.
This year I have stopped paprika and decided to just let Cinny be whatever color he ends up... which right now is a dark bay wonder what he will look like at the end of summer.