Definitely a question to talk to your CPA about but @Natalie Kolodij has provided some color on the topic. 

I once printed several thousand full-color mailers, only to find out that they bombed in terms of response rate.

That's within 5 days of delivery.I should also mention that he knows his numbers as everytime he mails he uses a different number so he can track everything from paper color, to if a logo is better or worse, even if the BBB logo helps...

Same color "ink".

Full-color on both sides or just black print on a yellow card?

And using colored ink is your best proof of a non-altered contract.It also indicates which document is the original in the event you have to produce the original document.

I think its a good way to stand out from the other mail a bit more, but I would figure out a way to make it pop even more.I think you need to admit to yourself that if you're bringing into question the color and wordiness then its something you can improve upon.My big concern... what's your company called?

the great thing about being an entrepreneur is that it's color blind.

We run a brokerage, rental office and an investment (we buy houses) organization so we're not as focused on a single type of seller as some of you, but at 33cents per huge/full color postcard...this is a major bargain.

You have to be pretty specific with them in terms of what you're interested in (styles, primary color, etc.), but they will give you something like 4 or 6 comps to choose from, and you can keep asking them to change stuff or give you new designs until you're happy.