  A sample of SiO-masing late-type stars located in the inner Galaxy is
analyzed with the goal of better constraining their obscuration. This reference
sample allows us to define mathematical relations between their dereddened
infrared colours and the observed colours (e.g. \Ks-[8], \Ks-[24]). The derived
equations define a property (a locus) of these late-type stars. Therefore, they
enable us to derive the interstellar extinction. With estimated spectral types,
it is possible to decompose the total extinction in the two components
(interstellar and envelope extinction). These relations are very useful to
classify extremely obscured late-type stars located in the inner Galaxy.
Estimating the two extinction components is performable on an individual
late-type star, independently of its surrounding, and also when a few
mid-infrared measurements are available.
