Subject: Why did our caterpillar turn fron tan to black?

Our class found two caterpillars on a green tumbleweed outside our 
school. The large one soon spun a cocoon in the dirt/gravel in the 
bottom of the cage. The smaller one has been eating well and growing, 
but over the past three days, it has gone from a tan with tan bristles 
to almost black with longer black bristles. What's going on? It 
appears to be active and healthy. 

Re: Why did our caterpillar turn fron tan to black?