Yellow Letter works nicely. I actually used it as a marketing campaign to find my lost ferret in my neighborhood... My ferrets kept running away for month after month, and my neighbors still bring them back to me due to that letter

I have also used it to generate response for my investing, and again it worked well. The envelope needs to match the letter though (not professional looking one).

Here is how I did it:

1. I purchased large number of yellow pads from HomeDepot
2. I ripped the pages out of the pads and stacked them up
3. I placed once plain white 8x11 paper ontop of one of the letters wrote my letter aligning them on the lines seen through the white page (an alternative is to just not align it on purpose and write in an angle).
4. Since notepad papers are 16# papers, regular printers tend to jam and you won't be able to run large numbers of them. My brother printer jamed every 5 to 8 pages while my HP printer jammed every 20 or so and I needed to print thousands
5. I went to Kinkos and used their $5k copy machines... I took out their paper and placed my notepad papers in the tray... placed my handwritten letter on the copier and ran a test copy to see the alignment.
6. Realigned the handwriting to the lines and ran the large print job. It jammed once or twice on 1000 or 2000 print job, can't remember.
7. I took the white paper I took out since I provided my own paper

I did not use blue ink, I used black ink so I can use the black&white printers.

Hope this helps