The correct shade of green for your tank should be Brunswick Green. You may find it difficult to match that shade of green outside of the UK if you ask for it by that name as I think it was specific to BSA (i don't know for certain). What you also have to remember is that as the paints were mixed by hand, batch by batch, that they wern't all exactly the same. The green on your tank dosen't look to far away from the original colour in your photo.