The Color White

Little Tom Dacre used to have white hair (before it was shaved), the naked children in the dream are white, and clouds are white. Whiteness in this poem is a symbol of innocence and childhood and contrasts with the blackness of soot, chimneys, coffins, and all the other bad things (adulthood, death, etc.) the poem mentions.

• Lines 5-6: Tom's hair used to resemble a lamb's before it was shaved. The speaker compares Tom's hair to a lamb's using a simile, which helps establish the lamb is a symbol of childhood and innocence. 
• Line 8: Tom apparently has "white" hair. This could mean he's just a blond, but white is also a symbol of innocence, purity, and childhood. He's not just a blond—he's a young one, who's being spoiled by his awful job.
• Line 17: The children are now white and naked. Finally, all that grimy soot is scrubbed clean from their bodies, and so they appear white. But this whiteness is also a symbol of their innocence, purity, and childhood. In Tom's dream, these kids are free to be just that—kids. 
• Line 18: The children rise on clouds, which are usually white. Sounds fun, right?