what RGB is blue?

[Keith Clarke]

Hi Tiemo,
Not a colour expert but it's an interesting problem - especially as 'blueness' may be subjective.

I visualise RGB values correlating with x, y, and z axis values of a colour 'sphere' where the origin is black and the surface, at cube-root(r-cubed + g-cubed + b-cubed) where r, g and b all = 255, is white.

Maybe you can define a section of that sphere's volume which is consistently blue in nature, given your target hues? Maybe the surface area of this section volume is defined by a slice (what is a 3D 'chord'?) by specific colours on the blue-red, blue-green spectra?

That's probably all rubbish but you never know? :-) 
Best,
Keith..
 
On 1 Jul 2011, at 10:39, Tiemo Hollmann TB wrote:

> Hello,
> 
> I am taking the mousecolor at different points from an image by script (not
> by clicking). I would like to analyse if the color I've taken is a "kind of
> blue", or another color. I want to change the backgroundcolor of an image.
> The background is always blue, but different blues and changing over the
> background. So what I want to do is to verify, what is background and what
> is foreground of my image.
> 
> 100% pure blue would be 0,0,255. But for a human being 25,75,130 (greyblue)
> is also still blue, but 240,20,180 is pink, though the third RGB value is
> higher as in my greyblue.
> 
> So I can't just check only the third RGB value, neither the sum or cross
> total. Has anybody ever heard, if you can define at all by math "what is
> blue"?
> 
> Any color specialist around here?
> 
> Tiemo