4.3. Colour-Colour Plots (CCP)
In case the far infrared optical depth is hard to estimate (no distance; no mass estimate from sub-millimeter flux density etc), then colour colour plot, CCP, is the only possible diagnostic tool available. A CCP is the locus of colour colour point as a function of 100, in that plane. After studying several combinations of the colour-colour plots, the most promising (i.e. with maximum diagnostic power of discriminating either the density distribution law and / or the embedded stellar type) are presented here. As a conservative practical criterion, only those CCPs are considered here, which change each colour by at least 0.7 dex over the entire 100 range (0.0045 - 0.34) considered here. The number of CCPs with good diagnostic value reduce as the stellar type changes from O4 to O7 to B0.5.