Test your (local) fonts here in a variety of sizes.
As far as I can tell, color / SVG-in-OT fonts are mostly only supported in browsers like Firefox. Commercial design applications, like the ones from Adobe, support it - I must wait a while before FOSS graphics applications do :(
So if you need to do something with them, do it here.
Update, 2019/06/17: Inkscape 1.0 alpha supports it nowBold? Italic?
.. .. .. .. sample texts:9px10px12px14px15px16px20px22px24px28px32px36px48px56px72px96px108px