Broccoli that turns yellow is still safe to eat and is not poisonous and won't cause you any harm but the Yellow broccoli won't taste as good as it would if fresh.
The broccoli turns yellow because of wilting from being left in the refrigerator for too long but no harm will be done to you if you want to ear the Yellow Broccoli but just be aware that it won't be as tasty and may even taste awful.
When broccoli has turned to yellow it's safe to eat but has also lost most if not all of it's nutritional value.