“If the values are wrong, the best color work in the world could never make the picture look true.”
Can you alter a value pattern? Lots of artists talk about doing it. Or moving a light object in front of a dark object. There may be ways of playing around with values in a realistic landscape, adding shadows here, eliminating shapes there, rearranging things, creating counterchange. I think here, that Gruppe must be talking about the precise relationship of values between distant objects and near ones, and between values within an object, so that the value relationships express a consistent quality of light within a scene.
In my personal experience, there are paintings that when I view them, the value/color relationships look spot-on. That isn’t to say that they are copied from nature (impossible, really, within the color space of paint), but that they are so internally consistent as to leave the impression of reality. Get even one of these relationships a little bit off, in comparison with the others, and the illusion falls apart. Gruppe here is saying that value is the most important relationship, color less important. It seems to me that color is an area where the artist can play a bit.