The French, until the later 1790's wore white, and some regiments wore white again from 1806-9, and again the French were in white from 1815-1820. However, the colour problem wan't that important when troops fought in formed units. As has already been pointed out, flags were good identifiers of nationality, and, at the range at which infantry would have been firing up until the 1850's, 100-150 yards, uniform differences, even with similar colours, would have been identifiable. Artillery officers would have been using telescopes, so could readily identify enemies. By the 1850's, infantry would have been in action at longer ranges, and were less likely to be in densely formed ranks, so, potentially, colour similarities may have been more important. There don't seem to have been many instances of friendly fire though, so uniform colours probably weren't as important as uniform styles. French infantry, in outline, would have looked different to Prussians.

I can't cite instances of it causing confusion, but I can tell you that even if the uniforms were identical, the flags were different. Infantry units had large national and regimental flags that distinguish them from those of other nations and units. These flags were extremely important for this reason, and losing yours would be a disgrace to your unit. Alternately, if you get the flag of an enemy unit, your unit would gain prestige. In fact the British Royal North Dragoon Guards (known as the "Scots Greys") retain as their regimental symbol a likeness of an eagle that sat on top of a French flag pole taken at the Battle of Waterloo in 1815.

Examples:

http://upload.wikimedia.org/wikipedia/commons/9/9a/Hohenfriedeberg_-Attack_of_Prussian_Infantry-_1745.jpg

http://www.1st-art-gallery.com/thumbnail/193776/1/The-Battle-Of-Ramillies,-The-16th-Foot-Charging-The-French-Infantry,-1900.jpg