Can we legally give more work to as-needed employees than full-timers?

Q: “I have a supervisor who insists upon scheduling PRN (as-needed) employees even during a work slowdown due to a low census of patients. My concern is that she gives more work to the PRN employee than the full-time equivalent worker who also works under her. I have a vague memory that this is not only a bad idea, but also an illegal one. Is there a Colorado statute that would support the FTE employee if we need to cut hours due to a low census?” – Kathleen, Colorado