Because state_color is not a valid field for button card. Check the docs.
Nope, I’m going mad.
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There is definitely something funky going on here! Not massively sure on how I would raise this as an actual issue?!?
Came here because I have the exact same problem. For lights and switches it shows the state colour. For groups it does not. I have 3 smart bulbs in my kitchen light fixture so I have those 3 bulbs in one group called “Kitchen Lights” that I use on a button card. Unfortunately it doesn’t show the state colour everytime it is pressed on or off.
Any ideas?
I looked into this back when @Martin_Granger and found that groups never changed icon color. Even before the state_color was added. So this is expected behavior. You’re welcome to make a feature request about it. But the current reasoning behind this is: Groups do not always have the state ‘on’ or ‘off’ (they can be any state based on what’s in the group), and because of this it’s never been handled.
Thank you @petro . I will look into making a feature request.
I wonder if there is another way or idea to get around this instead of making a group?. I am making a tablet dashboard so its nice to link all three bulbs to the one button. I don’t want to make a button for each bulb for the one kitchen light fixture.
was excited to see this. but too bad you can only make light group out of entities starting with “light.xxx”. i have sonoff to control my lights, and it all is considered as switch instead of light. so light group cant work.
turn it into a light
wow cool! is there anything that CANNOT be done in HA??? 
thanks @petro . this really helps OCD part of me that doesnt like seeing my group of light doesnt turn yellow when turned on.