Why is amber light on with battery showing at 99%?

The light on my power cord has turned 'amber' instead of the 'green' it normally is when I have the cord attached (which is most of the time), in spite of a high (99%) battery reading.

I don't recall this happening before. With a battery reading indicating a 'high' or 'full' charge in the status line, the light has always been green.

What is this signaling?

Thanks.