As long as the color is pretty standard like bronze you will find a lot of great options for almost all sizes.

Is this painted white or mixed with color?

Hi Roy...The mail piece type typically will dictate the length of the message, what is included or not and whether color, branding, even pictures of your family and pet is appropriate..Trigger words are important... 

Success depends heavily on which filters you used pulling your list and what you wrote in your message, the colors you used within the message and your calls to action. 

-Use yellow printer paper-Setup the page background with light blue lines to imitate  a yellow notepad (optional)-Use mail merge with your excel file of contacts to insert the prospects name at the top, and for the info you’ll print on the envelopes-Find and download a “handwritten” font to use in your document, or the option I’ve used is go to a site like YourFonts.com and turn your own handwriting into a font 👍🏻-Setup the printing size in Word for the envelopes you haveIf you’re going for the traditional yellow letter you’ll also want to have your font color as red. 

With the best vendors, you can get 2-sided full-color 8.5x5.5 postcards for about 60 cents (all costs considered - list + card + processing + postage).

I've got a list of about 4800 absentee owners - I believe I'll divide the list into 4 groups and market different size/color/message postcards to each list and just see if I can spot what works best.Have you tried different size postcards?

When I provide a postcard benchmark price, I should note that I'm speaking of 8.5" x 5.5" full-color both sides.

Change up the color of your envelopes depending on the season. i.e.: red for Valentines day or pastels for Easter etc. and last but not least keep your letter short and simple.

Some will say the "color of the ink" or the "color of the envelope" or "depends on what you say" Well the thing is it doesnt matter.