So it is supposed to make it look washed out. I see now. That's what I needed to know.
When you add a texture to another texture like that, and you exceed the maximum pixel threshold
IE
RGBA 0x78, 0x90, 0x78, 0x00 + 0x70, 0x80, 0x70, 0x00 = 0xE8, 0xFF, 0xE8, 0x00
It should tend toward 'clamping' the upper values. This can give it a washed out look unfortunately. If you wish to have a better method then you should use OGL's lighting model. You need the normals for each corner of the polygon then compute the brightness for each corner. Commonly guruad shading is used across the polygon to give the right look.
Of note NONE of the FF series 3d engines use light shading! If they did the models would look too dark very likely.
What are you trying to do? We might be able to help you get there with less pain! (maybe)
Cyb - Good fortune with your experiments!