Sorry, I posted the wrong link earlier. Here's the correct link:
http://www.kronoskaf.com/syw/index.php? ... Infanterie
They were very newly-raised at the start of the war, so it was probably just quicker, easier and cheaper to get them into the field dressed as a 'French' regiment. It seems that their light blue 'German' uniforms and fancy flags were authorised very early on, but not actually adopted until 1760 at the very earliest. The regiment also went through a major reorganisation at the time, being reduced from four battalions to two (it had three battalions at Rossbach in 1757). They were also brigaded with the French Royal Roussillon Regt at the start of the war, rather than in a German brigade, so perhaps they were initially classed as a pukka French (as opposed to Foreign/German) regiment?
One thing I've been unable to discover is the drummers' livery. Uniquely for a 'Royal' regiment, they didn't wear the Royal livery and were instead authorised to wear the livery of the Duc de Deux-Ponts. I've been completely unable to discover what that might have been, so went with Royal livery in the end.