Many roses change colors as the bloom ages.* If only a few of the blooms are doing this it could be a harmless virus that is causing it.* Do you know the name of this rose?* That would be most helpful to be able to research it and lean of it's characteristics.
It's often best to deadhead roses to keep the blooms coming along.* You should prune off the spent blooms to the first set of 5 outfacing leaves.* They should be cut and not pulled off.* The cut should be made at an angle to allow rain water to run off the cut stem.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.