20 mph advisory signs

West Northamptonshire have been placing advisory 20mph signs in (it seems) most of the villages in the area. These are ringed with green to indicate that the sign is not legally enforced, which would require a red circle. However, those people that are colour deficient, which get lumped together as red/green colour deficient, are quite likely to not be able to tell that the advisory sign is actually not a legally enforceable limit. Whilst this may achieve the desired result of making people drive at 20 mph, there may be an issue with the ambiguity of the signage for some individuals. A different colour to green would have been a wiser choice to avoid confusion. Approximately 8% of males have some degree of colour deficiency which could affect their interpretation of the signage.