There's a long-running tradition of illustrators and hobbyists confusing 'Prussian blue' the pigment with 'Prussian blue' the dye used by the line infantry of the Kingdom of Prussia. My understanding is that it the dye was probably indigo, like the French, and probably changed quite a bit depending on the period (and maybe even between regiments). I.e. the 'Prussian blue' of Frederick II's infantry vs that of Wilhelm I is not necessarily the same.
Getting back to the OP, there's even the likelihood that the infamous 'Prussian black' of Waterloo fame may well have been a lighter blue than the blue of the French. Personally, I'd be inclined to go with something like VMC/VPA Periscopes (maybe starting over VMC Dark Sea Blue) or AK Interactive Russian Blue Base & Light (for the sake of comparison). These all give a nice medium-dark blue IMO. For the Landwehr in particular, I'd say you could even go a bit lighter.
You could also just go with VMC Dark Prussian Blue and Prussian Blue. These are obviously referring to the pigment rather than the uniforms, but they work perfectly fine - it's what I normally use for my Continental infantry. If you do, I find you need to use the lighter Prussian Blue very sparingly as it can give unnatural-looking highlights.