Hi Mimi, because rhubarb is technically a vegetable, it doesn't ripen as such. The colouration will depend on how much light it's had - the greener, the more sunshine it's received, and the more intense the flavour, I think. The longer the stalk, the more mature it is and the stringier it is likely to be.
In case you haven't picked rhubarb before: Just pick the biggest stems by grasping them round the middle and giving them a twist-pull motion. The leaves can then be trimmed off and put straight onto the compost heap. It's best to avoid picking in rainy weather (such as is conspicuous by its absence where I live, just now), otherwise the stems will break.
Enjoy your fool!
:)
