Lemon turned broccoli purple
Normally, acids will turn broccoli a dull grayish color, and I don't recommend adding the lemon juice until right before you serve. For one thing the lemon juice will lose its flavor when cooked. 

Acids affect many, many foods when cooked — some for good and some for bad, so you do need to be careful when you add it. For example, acids added to beans before cooking will toughen them quite a bit. Acids added to proteins will denature them. And we all know what happened to Timothy Leary.