The colour could be AMT-7 - FS25190, FS35250 or A-28m - FS34533, FS25550 that was of greenish tinge and was used for painting of metal parts mostly. Also there could be used the A-18f or A.II Light Blue FS35526.
For the AMT-7 the Humbrol 89 matches almost perfectly. However it might have changed because the tinge of Humbrol enamels can vary among series of paints.
Also it is easier to use a lighter colour offered by a manufacturer than try to mix the basic one with some of white or light grey for instance. Just for the partical reason, you don't need to mix colours. I would suggest using the Humbrol 47 or Humbrol 65. But if you want to mix you can use the 19*Hu47+1*Hu147 or just 10*Hu34+2*Hu14 or 10*Hu34+2*Hu25.