I am painting a M113, and the interior should be painted with a color called "light interior". I don't know this color, it doesn't exist in neither Humbrol nor Tamiya. Anybody knows anything about this, maybe how i can mix it using humbrol or tamiya colors?

IIRC, some 113 interiors were painted flat white, and others a very light "hospital green" color. See www.kithobbyist.com/AFVInteriors/m113/m113b.html The LVTP-7 entry calls the color "interior pale green" Hth.

I don't know if it's the same, but the M113 we had/have in service with the Italian Army, including the first bunch that arrived from the States (the rest were built under license in Italy) had the interiors painted in a light green whose perfect match is the Tamiya XF-21 Sky.
I think that if you address it to Sabot here on Armorama he can be more precise.
Ciao

The official color is called Sea Foam Green, but Sky or Sky Type S depending on manufacturer is a good match. It is a mint green color. I sometimes add gloss white to Model Masters Pale Green until it looks right to my carefully calibrated eye. The interior paint has a slight semi-gloss sheen to it.

If it's an early, i.e. Vietnam period 113, I think the interior should be white. If it's more recent, Rob's post about sea foam (aka puke) green is the way to go.