I have a Wii Machine with docking station. When Red its charging up the remotes. When Blue, its means they have charged up.
You can always tell if your remotes are low, is when you go to the wii menu on your T.V. screen, look at the bottom and it shows how many bars are left on your remote. Some games will say remote bateries are low.
When placing the remotes in docking station, makes sure they are sat true to the contacts of docking station, i found if i didnt push the rubber sleeve up high enough, the docking station kept showing up blue. But when i pushed it true against the contacts, it turned red.