Typically darker down colored chicks mature in to birds with proper undercolor as adults. The standard calls for "delicate slatey blue" undercolor. Good under color results in good surface color, ie; good black markings where the bird is supposed to be black. It appears that your five week old birds' surface color is showing a bit light already. My light chicks vary a bit in down color from light smokey gray to darker almost black, some have a bit of smokey yellow. I'll try and find a pic.

