I am right now assembling the latest Humvee of Tamya with the Mk Grenade launcher.
The kit can be painted in desert or in nato colors.
If the Humvee is painted in nato colors the instructions tell you to paint the interior in olivegreen nato (tamya nato color)
I want to make it in desert ..the problem is the instructions do not tell whether the interior will be painted in sand colour or in olivegreen nato color.

What is the color of the interior of HUmvees which are painted in sand ?

I THINK (emphasis on the word THINK) that the interiors of the Desert Camo Humvees are OD- or whatever the original interior color is.

It depends on the original color of the vehicle. Some Humvees are factory painted in tan, and these will be tan inside. If the vehicle was originally painted in 3-tone NATO camouflage and only painted tan/sand before the OIF, than it will be forest green inside (not OD!).
The above is true for hard-top Humvees, like Tamiya's M1025. Soft-top variants would most likely be overall tan, even if they originally were 3-color, as the possibility to remove the canvas top makes it wise to apply the desert color also inside the vehicle. Also note that soft-top Humvee that is painted in 3-tone camouflage also has 3-tone camouflage pattern applied inside the vehicle! Unlike hard-tops which are overall FG inside.

Rgds,
Pawel