Apologies if this has been covered before...

I'm wondering why my readiness is down from green to amber (from 6 to 5) since yesterday, but my HRV went form 78.6 to 80.7 and my resting heart rate went from 59.3 to 53.9. Shouldn't my readiness improve?

No, your readiness shouldn't improve just because your HRV is climbing.  The best readiness is when  your HRV is in your personal middle range.  It is hard to tell what your personal middle range is based on the graphic you provided, since it only has 7 days of data.

A high (for you) HRV and low (for you) resting HR can also indicate your body can not rally to the occasion of training.  It could mean that  your body is swamped with stress and having trouble dealing with it by short term means such as increased hormonal output.  As such it may be trying other longer term means of coping, such as turning down it's response to stress altogether.

I happen to notice at times like that that it seems to take forever for my HR to get up to my target zone, and training seems like it generally takes a little more effort.

Had the same problem, thanks useful answer. +1

PS. New here only ~6 days of data.