In reply to crayefish:
Gripmasters are great for warming up but, as far as I can recall from what I've read, they are only of marginal benefit for training strength, mostly because the specific movement you are training (pinching/pushing you fingers against the middle of your palm) is not one that you replicate that often when climbing. Endurance-wise they may help a little bit but again, clenching and unclenching your fist every second is not something you do when climbing.
That said, I'm no expert, and they may make some difference. If you're getting bored with the red one then you may as well try the black one.