Yes you can but they are getting a bit tough now, especially after the long dry period. The colour doesn't make any difference. I had a look at mine yesterday and dind't see any that looked as if they would be up to much as they are getting rather tough and stringy. . There is a saying that you shouldn't harvest late in the season because the oxalic acid in the leaves goes back down into the stems but its not true.

I agree the colour doesn't matter. I have a red stemmed variety and a green stemmed variety. when the stems are wider than 1.5" in diameter I find they are a bit tougher. better fibre for us but less sugar too.