To summarize things: **Its all about the transformation of the value informations given by the rendered scene to the final sRGB display view.**

> My central question was: Why the color of surfaces desaturates in areas
> with higher light intensity?

**And here is the thing:** When transforming the scene-data to sRGB image-data, blenders ignores the fact that natural media (like films in analog cameras or the rods in our eyes) are sensitive to not only a small bandwidth of red/green/blue but more to a whole range of different wavelengths (to some degree). That means if , for example, a "red-channel-sensor/layer" is stimulated strong enough the although weak stimulation for blue/green will get so strong that the combined information will create an impression of white. In opposition to this in blender values for red will only influence the red channel, etc. - Therefore red surfaces will be displayed red, even if the light-intensity raises towards infinity.     

*For further information please visit this very helpful thread:*
*https://blender.stackexchange.com/questions/46825/render-with-a-wider-dynamic-range-in-cycles-to-produce-photorealistic-looking-im*