Hi Ninanoki
I also suggested Lab color so will respond on that as well.
Lab allows you to separate out Luminosity from the "a" and "b" color components RGB does not.
Doing math with just RGB gives way too much weight to the blue component as for the same color number, blue actually has about 1/6 the luminosity impact as green and about 1/3 as much as red. So not weighted correctly on how our eyes perceive color
Also, with RGB you run the risk of having a palette in one color space and an image data in another color space if you are not careful. That can't happen in Lab.
The Lab color space numbers are based on a scale that more or less evenly represents how our eyes perceive differences in a color. RGB is not.
I think you will have fewer problems and better matches using Lab and not RGB. There are possible workarounds yet why mess with that.
As far as Excel. to know the difference in colors you want to know the distance between two points in 3D space. That is using the approach from my prior post where you take the RMS value of the differences between the two color numbers in each channel. Best to do in Lab yet could be done in RGB (blue in the RGB channel can throw off the accuracy of visual differences so still suggest Lab)
Adding the numers in Excel does not make sense. You could actually have two totally different colors add up to the same number and that is not the result you want.
Hope the info helps.
ADDED EDIT: Here is a link to convert from Lab to Lch
http://www.brucelindbloom.com/index.html?Equations.html
