The two restrainers potassium bromide and benzotriazole can be used to adjust the print tone. Adding bromide to the developer warms the tone while benzotriazole cools it. What I personally find most attractive is a pure neutral black.
There is an optical illusion caused by the eye and how it focuses different colors. When a print image is warm it seems to float above the paper and when it is cool it appears to be under the paper surface. When the image is too cool this effect can give the print an unattractive veiled appearance. Japanese sumi painters take advantage of this illusion by using two different types of charcoal to make ink; one slightly brown and one slightly blue.