Understanding the color coding of a weather map
Unfortunately it's common for satellite data services to show these types of images qualitatively, omitting the units or even a colorbar.
This is an infrared image from the 10.8 micron channel (TIR1) of INSAT-3D.  That website you link to is a bit misleading because (for me at least) following the link puts the text describing the 0.65 micron visible channel under the image.  If you click &quot;Infrared&quot; in the sidebar of that page the text is replaced with the correct text about the 10.8 micron channel.
Given that TIR1 records are stored as 10-bit values (i.e., in the range 0 to 1023) and the color scale maxes out at 939, I suspect that they are just plotting the raw count data.  This is normally transformed into radiances by applying a linear scale and offset to put the data in physical units.
I had a quick look at the equivalent 10.8 micron image from Meteosat over the Indian Ocean (see below) and I get a similar count range and a similar image when I apply their color scale.  I also suspect that they've inverted their data, e.g., plotted count_inv = 1024 - count, to fit with the viewers' expectation that cold clouds appear white.