Edit: current situation in a nutshell:

CBDT spec proposal stated no glyf table should be present in CBDT/CBLC fonts. Current spec doesn't say anything about this. Fonts built on the proposal specs will fail in browsers with OTS, as the absence of a glyf table makes OTS reject them. In current FreeType implementations (Chrome on Android), these glyfless fonts work, and the color glyphs are shown.

For CBDT/CBLC fonts with a glyf table the glyph in the CBDT/CBLC table will be ignored, effectively always rendering the normal glyph, making it impossible to get the color glyph.