<p>A colour-colour diagram (CCD) like this
<a href="https://i.sstatic.net/lSwev.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/lSwev.jpg" alt="http://www.brucegary.net/All-Sky/"></a>
compares the difference in magnitudes at different wavelengths. The trend in the pattern of colour-colour comparisons tells us about what type of source the light comes from.</p>

<p>Is there any significance in the best-fit line of the trend? As in, if I did a linear regression of the left plot above, I'd get a gradient of (V-R)/(B-V). I realise that this ratio doesn't have any physical significance in terms of a single source, but <strong>is there any value in calculating the best fit line and its slope or regression coefficient? Does it tell us anything about the group of sources it's associated with, or is it just used to calibrate filters</strong>, as it states on <a href="https://en.wikipedia.org/wiki/Color%E2%80%93color_diagram" rel="nofollow noreferrer">Wikipedia</a>? The answer to <a href="https://astronomy.stackexchange.com/questions/25813/what-is-equation-of-straight-line-for-a-ideal-blackbody-on-the-color-color-diagr">this question</a> doesn't really answer mine.</p>

CC BY-SA 4.0
2018-09-25T11:32:06.893Z
Yes, it does. For example, globular VS open star clusters. Stars in an open cluster are younger and bluer compared to ones in a globular cluster, therefore the CCD tracks are different.

CCD has a lot of applications. Check Wikipedia: https://en.wikipedia.org/wiki/Color%E2%80%93color_diagram.
Kornpob Bhirombhakdi
CC BY-SA 4.0
2018-09-25T22:24:04.840Z
@KornpobBhirombhakdi Thanks, I know about the applications of CCDs and the different groups that sources make when compared; that's not quite what I'm asking. I'll edit the question to make it a bit clearer.
Jim421616
<photometry><magnitude>
Regression line in a colour-colour diagram
Jim421616
# Regression line in a colour-colour diagram

(Asked by: username_0 on 2018-09-24)

A colour-colour diagram (CCD) like this
[![http://www.brucegary.net/All-Sky/](https://i.sstatic.net/lSwev.jpg)](https://i.sstatic.net/lSwev.jpg)
compares the difference in magnitudes at different wavelengths. The trend in the pattern of colour-colour comparisons tells us about what type of source the light comes from.

Is there any significance in the best-fit line of the trend? As in, if I did a linear regression of the left plot above, I'd get a gradient of (V-R)/(B-V). I realise that this ratio doesn't have any physical significance in terms of a single source, but **is there any value in calculating the best fit line and its slope or regression coefficient? Does it tell us anything about the group of sources it's associated with, or is it just used to calibrate filters**, as it states on [Wikipedia](https://en.wikipedia.org/wiki/Color%E2%80%93color_diagram)? The answer to [this question](https://astronomy.stackexchange.com/questions/25813/what-is-equation-of-straight-line-for-a-ideal-blackbody-on-the-color-color-diagr) doesn't really answer mine.

**Tags:** photometry, magnitude
> **Comments:**
> **username_1**: Yes, it does. For example, globular VS open star clusters. Stars in an open cluster are younger and bluer compared to ones in a globular cluster, therefore the CCD tracks are different.  CCD has a lot of applications. Check Wikipedia: https://en.wikipedia.org/wiki/Color%E2%80%93color_diagram.  
> **username_0**: @KornpobBhirombhakdi Thanks, I know about the applications of CCDs and the different groups that sources make when compared; that's not quite what I'm asking. I'll edit the question to make it a bit clearer.  

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