When the previous warts were removed, did they confirm they were non-malignant warts?
Sorry I've taken such a long time to get back with your answer.
If these warts were confirmed non-malignant the first time round I wouldn't be worried by then now even if they are changing color.
Often over time, extra pigment can develop in cells that are abnormal eg wart cells. Hyperpigmentation could develop after ongoing low-grade inflammation and is a normal process.
If the warts are not bleeding or irritating your poodle I would simply observe them and not rush to have them removed again.
Hope this helps.