My male hadn't eaten in 5 months(1000g wall) and someone told me to try an all-black f/t rat. I did and he attacked it like it owed him money.
I don't understand exactly why it worked, but it did. The next week I tried another black and it refused....so I tried him on a white one that my others refused, and he took it. So maybe the black got him started again...maybe it was just his time to eat...who knows ?