The tone of Shel Silverstein's poem "Masks" is one of regret.
In the poem, a boy and a girl walk through life wearing masks. In fact, the drawing which accompanies the poem shows the masks to be ridiculously oversized and hiding their entire bodies. These masks also hide the fact that both the boy and the girl have blue skin. Because they are different from the rest of society, they use the masks to hide their true color in order to fit in. However, at the same time, they yearn to find someone like themselves, so "they searched for blue their whole life through." Regretfully, because the masks hid their appearance, the boy and girl could never see each other's color and recognize what they had in common, so they "then passed right by -- and never knew."
The blue can be literal and figurative. Along with the children both being literally blue in color, they could also have been sad and dejected for not being able to connect with another like-minded person. In either case, they missed their chance to connect with who could've been their soul mate because they were hiding their true selves and pretending to be what they weren't. The speaker recognizes this fact and, therefore, his tone is regretful.