Your ADSB-out signal changes to different data once it 'detects' that you are on the ground (or vice-versa).The change over can be initiated by a squat switch if you have one, but for most RV's it's a combination of ground speed (gps) and IAS (pitot system), e.g., if well below stall speed you must be on the ground. The red data (air-ground) indicates that the FAA believes you were still sending the "airborne" data when you were in fact on the ground. Now, I have no idea how the FAA actually determines this. At my home airport (KLVK) there is no radar coverage at ground level (despite being within the SFO class B airspace). Is that the issue for me? At KLVK I also got a 'red' air-ground report while always passing at other airports. I played with the airspeed parameters a bit, then landed at LVK, hard breaking to a full stop on the runway. I passed. So there's something not quite right with how the testing is done. But I passed, and continue to pass, so I don't worry about it anymore.
Edit. I should have mentioned that I'm using a Trig TT-22 mode S (ES) transponder for adsb-out, and my GRT HX shows when it's in the ground mode, and every time I look it's in the correct mode. But of course I'm not looking at the panel at the most critical time: just after lift off or just after touch down. My feeling is that the FAA's tolerance on the switch over is pretty tight.