There's a France 1814 Guide, which has plates have French & Prussian (the next force that Warlord plan to release) uniforms from that year. Would those color schemes work for units that fought at Waterloo? Anything I should look out for?
Yes, they'd mostly be the same. A minor thing to watch out for would be that both armies might be slightly better equipped in 1815 than in 1814. There'd been a year of peace in which supplies could get from depot to front line.
Another thing to watch out for was that between 1814 and 1815, the Prussian former reserve and various free and foreign corps were taken into the line and numbered 12 to 31 . It appears their attire didn't change much in the time available but the numbering did.
would the facings/lace/etc references still be useful for 1815-era figures?
Yes they would – with the proviso that few units in the Peninsula were also present in 1815. So while the facings didn't change, the units involved did. Here's a list of every regiment:
link
You can't go far wrong with yellow, green or dark blue, which were the commonest colours by far.
Wikipedia has a good order of battle for Waterloo so if you have a scan of that it will tell you what units were present.
