Roglite red. This gives a very bright red light that can be solid or flashing. The unit itself seems quite robust, The clip that attaches the Roglite to the collar is a good strong one. Slight downsides are that the on/off button on the front needs very firm pressure so isn't very easy to use, and the whole unit is quite heavy. Only time will tell how long the battery will last. I'm trying this one as it looks more robust than other flashing led gadgets for my dog's collar have been very disappointing.
Great lightweight light for my black dog. Company has great products and fast service.