other areas (not foliage) showed values between 0.2 and 0.9 and I don't know why, since there is no photosynthesis occuring there.

Getting high NDVI values doesn't require photosynthesis, it just requires a big difference between the values in the blue and red channels. In natural areas, few surfaces (other than leaves) have a color with a big enough VIS/NIR difference. But an indoor photo that has been manipulated to exaggerate the red channel could have lots of pixels with such a difference.

Are you capturing RAW images? Lots of good single camera NDVI photos have been taken with an artificial custom white balance that exaggerates values in the NIR channel. I have not been able to recreate this exaggeration after the fact if it was not done while the photo was taken. If RAW images are captured, it should be possible to recreate the adjustment, but adjusting a jpeg photo that has already been manipulated by the camera firmware probably cannot reproduce the effect of an in-camera custom white balance.

The post processing adjustment you are making has not only increased the red values, it has also decreased the blue values. Many blue values are near zero even in areas that are not black. When computing NDVI (for a blue filter: (R-B)/(R+B)), as B approaches zero, NDVI approaches R/R which equals 1. So many pixels with low B values are producing NDVI near 1.0 (magenta). This might be an artifact of the way your color adjustment was done. Maybe a different color blue paper would produce different results.

Also, calibrating the process under indoor light with non-natural surfaces might not transfer well to outdoor situations.

Chris
The KAPtery

@carolccarvalho please your help, I trying to do a similar project for school, but I'm new with MATLAB program, Could you send your codes to implement with my project?

Also could be interesting to @maggpi -- check it out!