Why? Shades of grey define everything.
As painters, we often forget that yellow is a light hue and can never be seen as yellow and dark at the same time, no matter how we may try to manipulate the color itself. Blue on the other hand can be almost as dark as black and as pale as snow in shadow. Yellow has a very narrow value range and blue has a wide one.
As a novice painter, I often lost track of the values until I discovered the quilters’ best friend – the value finder: clear plastic rectangles in green and red. Red is used to discern value in cool images, those in shades of blue and green. Green value finders are used for images dominated by the warm colors, reds, oranges, pinks and golds. My paintings are usually a mix of warm and cool colors so I usually use both value finders on every image.