I believe T. unistriatus is usually between 9 & 12 mm in length, according to Downie & Arnett. This will probably be a different species if your length of 6 mm is correct.

Unless you use a closeup flash attachment, your camera's sensor will create a setting suitable for the average tone it sees, in this case a very pale greyish color. To get it to create settings that are right for a dark beetle such as this one, I've found the best way is to place the beetle (sans tag) against a same-tone background, which could be red, maroon, green, blue or a very dark grey. Give it a try and see how it works for you.