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".

Would you mind sharing how many you usually have printed at a time, full color or not, size and what the cost is, including mailing?

Assuming budget isnt an issue1) Full Page Color Newspaper ads in Sunday's newpaper front page of the Classified section...2) 30 minute Infomercials ran on single digit channels at 6:30pm Drive time.3) 60 second TV commercials at 730am news drive time most popular network station4) Per 250k people 12 Rotating billboards, rotated ever 45 days in the neighborhoods you buy in.5) Direct mail 16 letters per prospect to your prospect base...How about the most expensive methods of advertising...

How about the color of the ink or the font or the size of the envelope if we are going to stay to just the letter..

I would like to make my own yellow letter using a color printer.

Just for your information there have been studies done as for colors for signs.

What color did you use and what was the verbiage? 

I print them on my HP 2025 color.