Ahoj,
on a structure that is predominantly made of glass, which color=* tag to apply?
I would say color=transparent, but is this OK, or discouraged? The wiki on the color key does not say anything about transparency. So should I instead take the colour that the non-glass elements have?
Example: This public transport shelter:
color=transparent or color=brown?
See also 
this OSM note and it’s discussion.
Seems to me that the color of the shelter is black and has a side wall that is glass so 2 elements: roof and wall. Roof will be color black, material metal; Wall will be a way with material glass.
If we’re micromapping to this level, also consider adding the advertisement location!
Transparent is not a color, it is more a property of a material. Although in the digital context a coding (hex, rgb,…) is assigned to the absence of color.
I think the correct answer is more related to the material of the building.
building:material=glass
building:material=mirror
This is not a real building so I suggest:
material=glass;metal
Others have mentioned more intuitive alternatives for this particular case, but I just wanted to clarify that the documented value range for colour=* and related keys is either an RGB hexadecimal triplet (e.g., #FFEFD5, #ffefd5, #fed) or a CSS named color (papayawhip). Notably, transparent is a valid keyword value for the CSS color property, but only as separate syntax. At least as far as our documentation is concerned, values like rgb(255, 239, 213) and #FFEFD5AA are invalid, but this hasn’t stopped some mappers from trying.
I resolved this by adding colour=brown (it is not black) and material=metal;glass.
Regards!