Last night/this morning at about quarter to one, I began to wonder about a Tunisian crochet mitered square in two colours, as you do. Because I was too lazy to get out of bed just then, I decided to make such a square and see whether I liked it. Fortunately, my Tunisian crochet hooks are in a drawer of my bedside locker and I had a failed experiment in a bag that was in reach, so I ripped that back and began to crochet. The yarn is cotton, so I figured that I could use the end result as a washcloth/dishcloth. I ended up ripping out the first one because it was too small, but it was enough to show me that I liked the end result, so I made a longer chain and did another one.
You could do a bunch of colours, too, or several rows in one colour and several more in another or a solid square or whatever you want. I will probably use this with some smaller scrap bits and have random colour changes--quite easy to weave in ends with Tunisian crochet!