There should never be anything near medium blue on French uniforms. Indigo fades, if it does at all, to gray or light purple, not bright blue.
Here's an actual habit-veste: link
Campaign look is also to gray:
Dark is the way to go:
You will continually find that in every defense of medium blue are three claims:
1) Uniforms fade. But people exclusively do this for French and French alone – never Russians, never British, never Briish Rifles. Only the French get this treatment – why? Partly because it's a hard color to pull off with the ubiquitous Vallejo Prussian Blue or Citadel's Ultramarine Blue for space marines everywhere. Start with black and highlight with Navy if you must.
2) Dye lots were not consistent. Again, people exclusively do this for French and French alone – never Russians, never British. Every army is treated ideally, except for French where one must invent fanciful colors to represent them. When indigo was in short supply, Napoleon switched to white in 1807 until it's resupply was assured.
3) You have to take into account distance and therefore lighten them more than you ordinarily would. Again this only applies to French, and further, only to French Napoleonics. Never hear this advice in Ancients or ACW, or WW2 wargaming forums.
Assuming they are correct, one can at least "fade" or lighten in the right direction. Moving to medium or sky blue is probably never correct, not even once for indigo which, by Imperial decree, had white threads woven into it – not dissimilar to denim or chambray. Fading and sun just does not get you where people want to frequently take it. Rather highlight up with gray.
I say go subtle and keep it dark:
Incidentally, the final in the World Cup – the French uniforms were the same color blue (sans the shoulders and arms). Look here for highlights: link
Here's my recent pic of French Engineers:
My current and preferred recipe seen here on the coat of this officer is (all from Reaper Master Series):
Shadow: Nighshade Purple
Base: Midnight Blue
Highlight: Twilight Blue
Look at reenactment sites to see what they may look like in the field. Go dark:
Even in sunlight – to gray – not bright blue: