Here is a picture of gooseberries, some ripe, others not. The ones without prickles are purple when they're ripe. They are super sour when they are green and taste a little like grapes when they are ripe.
The sad thing about berries is that they do not ripen once they've been picked. So if you pick them too early, you will have a bunch of sour berries. This has been a tough lesson for my kids to learn. The second they see color, they want the berries right that instant. But trust me, they are worth the wait.
Grapes are the same way, but you don't tickle the branches. They may be purple for a long time (or green if that's the color they're meant to be) but still be sour. You need to taste the grapes to see if they are ready then cut the whole cluster off.