What is turning yellow, the blooms or the leaves? If it is the leaves, then check the soil, they are either being under watered or over watered. They may also be root bound (meaning that the roots have filled the put) and they want to be transplanted into a larger container.

If you are talking about the blooms, do you know the name of the varieites? Where did you purchase these plants? How long have you had them? Some roses change colors, especially in the heat.