Short version: Color/plastic is really not that big a deal and the $100 difference from the 5c to 5s is significant.
Long version:
In all seriousness, does the plastic shell make any true difference to the majority of people (not the majority of people on this site, but the majority of people that are not reading a tech-centric chat board)? I did a couple of quick searches, nothing scientific. I found several reports that estimate that ~75% of smartphone users have their phones in cases. My again unscientific eyeball test tells me that as well.
Once an phone is in a case, the color and material of the backing is or can be virtually irrelevant. Yes, I understand that some cases have holes to show through the underlying color and even the printing and find that those holes, more precisely the lower few rows of holes, make absolutely no sense. But there is a humongous industry in aftermarket accessories that has been fueled by phones/tablets. And with the new colored backs, you'll see many more creative cases that exploit the coloring, along with cases that cover the entire phone back that hide the color. Ultimately, the color can be as exploited or negated depending on preference.
I have a black iPhone 5. It has been in a case since the day it was purchased. The back could be purple plastic, and I would not know the difference. I actually think that the black face is what should be the bigger put-off to people as many prefer the white face, and once in a case that's what mostly remains visible anyhow.
Onto the pricing. I see many posts indicating that the 5c makes no sense, spend the extra $100 and get the 5s. That is very very likely what I would do (again, coming from someone on a tech-centric chat board). I would want the flagship product.
But step back, don't think about a singular phone for a single individual. Think about the incredible cost of phones and plans, a cost that simply didn't exist in the past for families. Based on my age and position in life, I have many many family members, friends, and neighbors with 2 and 3 teenage kids. When shopping/upgrading phones/plans for a family of 5 with 3 kids, wow it can add up, to the tune of several hundred dollars for the phones and $300ish a month for the plan. 5-10 years ago, that same family was spending $70/mo on their basic plan, and getting new phones for $0. Now, we're looking at monthly costs that rival a car payment. A $100 difference in phones is not insignificant and will drive sales.
So why did Apple not just make the iPhone 5c a freebie? Value perception. I am in sales, selling enterprise-level contracts to large corporations that can exceed $500,000 in total contract value. The worst thing I can ever do is provide something for free. It completely undermines the value of my product and service, and the client or prospect never will look at me the same.