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 now

Bold? Italic?
.. .. .. .. sample texts:
9px
10px
12px
14px
15px
16px
20px
22px
24px
28px
32px
36px
48px
56px
72px
96px
108px