Based upon what was said about being New Carpet and same roll and so forth, I do not believe the term 'color change' would be correct.,(unless the term is being used very loosely). When you get somewhat of an overview, especially in the downstairs fill piece (? I think that was what was shown), the color on both sides appear very close. While there can be a lot of color drifting to the edge, many side match problems occur due to 'Texture Variation', which can be due to tension and or shearing effects. This will make a carpet with the same color appear to be two completely different colors all due to the light reflection. This is what I see here, from the photos. Flood that carpet with light and look straight down onto the seams and I would tend to think one could not tell one bit of difference in 'color'. The fact that the same side stays dark regardless of which side it is viewed from also helps to point this direction, IMO.
Of course being there and seeing it in photos is quite different.
Texture Variation is simply the difference of pile height or tension, in most circumstances. A slight difference of pile height will make the pattern look different and make the color appear different, all due to tension and pile height. When you look at the carpet downstairs, you can see the depth of the pattern is quite different and more defined on one side, it has more depth to it where the other side seems to 'lose' some of the pattern, since it is not so defined. This is classic to what is referred to as 'Texture Variation'. This condition makes the color appear different due to the light and how it is reflected off the face yarns. It is a form of an optical illusion.
Just my two cents worth.
SA