I think they meant the Duck Egg Blue colour. As Airframes said the Sky Grey paint is the US name of the dope used for planes that were ordered by RAF in the USA rather and was never used for land planes made in the Great Britain. The DEB colour can be found in old publications and is said to be used during the early period of the war over the Great Britain. According to the info there, the paint was slightly lighter then the Sky type S and just bluish contrary to the Sky type S that was greenish. Officially the Sky type S was introduced on 6.06.1940. and was used for undersides until 15.08.1941 when the using of the Medium Sea Grey was ordered.
The colour was discussed here and all around the net many times. But there is no definitive answer to the question whether the DEB ( Sky ) is the same name for the Sky type S or not. However it seems that the colour could be used for repainting of fighter planes with the old scheme for undersides ( eg. Silver/ Day-Night) when the Sky type S was introduced but not delivered to squadrons yet due to its shortage.. So it had to be obtained by mixing paints by the maintenance crews what resulted in many shades depending on the ration of colours used for the mix..
Anyway I still agree with Terry that the Sky type S is the correct choice.