This is very cool, and I have a suggestion for an enhancement: you're hosting the sliced up images yourself, so you should be able to load them into a canvas element and analyze the colors. If the user switches to "solid color" or "custom html", you can set the background color to the most common color along the border of the region. You could even set the default text color to the brightest/darkest color within the region as well.
We've been looking into color automation for exactly this. Ours will be a combination of canvas and server-side.
Server side to do almost exactly what you're saying. Automate the background color picking and even detect if the cell is just a block of color (and set it appropriately). We're doing this server side so it's available to all users, not just users with canvas.
Then on top of that Mailrox could benefit from an canvas based eye-dropper tool for picking out text colors. I hadn't thought of making the system guesstimate the test color but that's a nice idea in addition to a picker.
