I want to apply for a program where I will be doing research in a particular area for roughly 6 months. This is an area of research I'm very deeply interested in, but I don't have any experience yet. The program asks for a letter of recommendation and I'm a little torn who to ask to write this letter for me. I'm currently a visiting assistant professor at a college and I have a colleague in my department who knows about my interest, but he's honestly someone who isn't super familiar with my work. He's offered to write a letter with the caveat that this would be a relatively weak (albeit positive) letter. On the other hand, I'm considering asking my thesis adviser to write a letter for me. My concern however is that this would like like I'm relying too much on my grad school experience to get me into this program (it's been almost 2 years since I finished my PhD). Unfortunately neither one will be able to speak that much to my experience in the intended area of research (since I have none). Are there merits to relying on a newer colleague at this juncture, or is it safer to rely on someone more familiar with my work?
You want letters from the people who know your work best. At this point that is probably your thesis advisor, with whom you have actually done research, even though in a different area. Be sure to explain your new interest when you ask your advisor for that letter.