All yellow letters or mix it up?

[Trevon Peracca]

I'm looking to start campaign number three here in the next week or two. I've seen people who are exclusive to yellow letters, but I have also seen people who like to mix it up and use a more cost-effective strategy. By that I mean on the first touch could be a postcard, the second touch could be a letter, the third touch could be a zip letter, fourth can be another yellow letter and fifth can be a professional typed letter with company logo. Anything after the fifth letter can just be a postcard.

I'd like to hear some thoughts on this. Which is most effective, and which is most cost-efficient? I'm in the west suburbs of Chicago.