My Japanese are in their blue winter uniforms, rather than their white summer uniforms, for the simple reason that this allows me to use them for the Russo-Japanese war aswell.
The standard M1886 blue winter uniform is worn by my army. It was in service up until 1905 (and beyond even that date for second line regiments).
If you are portraying Imperial Guards, the yellow piping on hat and breeches should be replaced with red. The yellow stripe on the cuff of some troops represents senior privates; two or three stripes are NCOs. Officers have black piping on cuffs and collars and black chest-loops, but this is invisible at 15mm scale so I have not painted it.
Some officers wore a mix of blue winter and white summer dress, or even just the white summer dress right up to 1905, so you will notice some figures painted in this way. Officer rank is denoted by the number of black bands on the hat, plus other details invisible at 15mm scale.
The cavalry also wear the M1886 uniform, with "Attila" jackets. These had yellow hussar-like loops.
Cavalry troopers wore red trousers with a green stripe, and had green piping. I could not find any reliable information on saddle-cloths, so I just used common sense.
My figures are almost all "Old Glory", from their Russo-Japanese War range. Some staff officers, however, are conversions from Essex Napoleonics.