Residential areas and wetlands both have a grey-ish colour. The shape and proximity to water might give away which is which, but it might not always be clear. See this part of the map f.e. http://osm.org/#map=17/8.67642/-12.87395&layers=H
Since wetlands are conceptually between a water body and a forest or grass field, I guess their hue should be something between blue and green.
Residential areas and wetlands both have a grey-ish colour. The shape and proximity to water might give away which is which, but it might not always be clear. See this part of the map f.e. http://osm.org/#map=17/8.67642/-12.87395&layers=H
Since wetlands are conceptually between a water body and a forest or grass field, I guess their hue should be something between blue and green.
Maybe an effort could be made to differentiate between grass-like wetlands and wood-like wetlands, see the distinction on the wiki: http://wiki.openstreetmap.org/wiki/Key:wetland
Thanks for considering this request.