As you surmise, lack of sunlight. Still edible though.

Thanks OB

there are also green stemmed varieties. and the pink stems you get in supermarkets have been forced in sheds so they are pink and quite sweet.

The colour doesn't affect anything but looks Hank. Just remember not to overpick it this year...

I’m eating them red and green, I just put extra sugar in and it tastes fine, and I haven’t forgotten S.

If you want to avoid all that extra sugar you can use a little bicarbonate of soda after the rhubarb is cooked. Sprinkle it on and stir gently to spread it. You will get amazing amounts of froth as the alkali in the soda reacts with the acid in the rhubarb, and when it all calms down you'll find you need very much less sugar. I've been doing this for years ever since my OH was told to avoid sugar. Start with perhaps and eighth of a teaspoonful (depending on the amount of rhubarb)and take it from there according to taste. Its nice to find another rhubarb addict...have you tried putting two or three halved strawberries in before you cook it?