How to reduce number of raster bands of a RGB picture?

The Green-Red Vegetation Index is a useful one for ecological purposes.

From the following publication:

Motohka, T., Nasahara, K. N., Oguma, H., & Tsuchida, S. (2010). Applicability of green-red vegetation index for remote sensing of vegetation phenology. Remote Sensing, 2(10), 2369-2387.

We chose GRVI because the response of GRVI to various ground covers may be simple to interpret. In terms of the balance between green reflectance (wavelengths of around 500–570 nm) and red reflectance (wavelengths of around 620–700 nm), we can find three groups of spectral reflectance patterns for major components of ground cover (Figure 1):

• Green vegetation (conifers, deciduous trees, and grass): ρgreen is higher than ρred.
• Soils (brown sand, silt, and dry clay): ρgreen is lower than ρred.
• Water/snow: ρgreen and ρred are mostly the same.

Vegetation has positive values, soil has negative values and water/snow has near zero.