I did a little experiment last night, creating two rectangles, one  of 
hue = 240, saturation = 90%, and value = 90%, the other differing only 
in saturation, = 50%, i.e., reducing saturation by 40%.  Then, from the 
Hue/Lightness/Saturation dialogue, I increased the saturation of a 
selection in the less-saturated rectangle by 40, thinking the colours 
would then match.   They don't.  In fact, /no/ adjustment of saturation 
from that dialogue will make the colours match and, further, adjustment 
of the saturation also affects the colour value.  Adjusting colours that 
differ only in value (which I assume is the same as what's called 
"lightness" in the H/L/S dialogue) yields similarly unexpected results: 
it was possible, using value adjustment alone, to adjust a colour with 
an HSV triplet of [240 90 50] to [240 90 90], but only by increasing 
"Lightness" by 61 rather than the expected 40 (or perhaps the at least 
somewhat intuitive 80, the percentage increase of 90 over 50).

I confess I don't understand colour, but is my understanding even poorer 
than I thought?  Or is this an area in which GIMP needs a bit of work?

(The utterly prosaic background of this question is that my wife runs an 
eBay business for which I am the reluctant photographer.  Frequently, I 
take multiple pictures of the same object, under the same lighting, 
differing sometimes only in the distance of the object from the light.  
The resultant colours  in the photographs are frequently significantly 
different and I use  GIMP to try to make them match.  This is remarkably 
difficult to do and if I ever get the time I may undertake to write 
something for GIMP to make this easier.)  (And I won't even comment on 
the near-impossibility of getting the image colours to perceptually 
match the in situ colours...)
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