Nope. Lightening the 75 will make it go more towards what you need, with a hint of yellowish brown - the 'bronze' effect. If you lighten the 78, it'll go either very light, wishy washy green, or more to a grey with a hint of green!
If you can't get the 75, try either Humbrol 30, or Humbrol 163, the latter being satin, and actually a better match for Dark Green, as the Humbrol 30 seems a bit bright of late. Add a spot or two of 29 Dark Earth, and a spot or two of yellow, until you reach an acceptable 'scale' shade' If you can't get them in Humbrol, then the nearest equivalent in another brand, and adjust as above until the desired shade is achieved.
Basically, the base colour for the green is the colour used for RAF topside camouflage, adjusted as required, and you might need to add just a small dash of black, to darken (sugar to taste, a slice of lemon, and salt on the rim!).
Bear in mind you'll need Dark Green for the camouflage schem topside.