Well, light grey overall is a pretty safe choice. Slightly more seriously, I've built 4 TIDS and all four were grey, they just don't look quite right to me in any other scheme. That's not as dull as it sounds though, you can make a model like this much more interesting by using several slightly different shades of grey to bring it to life, working tugs were constantly re-painted, and old paint never quite matches new. As they were built for war work in WW2, all were grey originally, but most would have been repainted for post war civilian service, most probably with black hulls, brown superstructure & dull red below the waterline, and perhaps something slightly more exciting for the funnel. One thing that a lot of modellers forget, is that paint was hard to come by in the austerity times that followed WW2, so if you want something with an eye catching colour scheme, this type of vessel probably isn't the best choice.