Colors have their own natural weights. Yellow is a lighter weight color, while blue and red are heavier.

Looking at this chart created by Munsell, we can see how the weight of the color relates to values in a black to white value scale.

Yellow is a naturally lighter color, while blue and red are naturally darker.

The image at right gives you an idea of the color wheel converted to grayscale.  This gives you an idea of the relative value, or weight of each color.

This is not a scientific study, it is just to give you an idea.