The best I could do was match my color chart to the article in Jan. 2003 Air Forces Monthly - it looks very close to Testors Model Master "RAF Medium Sea Grey", no. 2058 enamel and no. 50121 acrylic. The cross-reference I have does not include Humbrol (unfortunately). These are the equivalent colors it does list:
Floquil Polly Scale Acrylic #505258
Floquil Polly S Acrylic (nope, not sure what the difference is) #500847
Tamiya Acrylics - XF20
Gunze-Sangyo Acrylics - H335
Hope this helps....