I believed that the higher HRV, the better. Today, I got 102, but the colour was RED. I don't understand why it tells me that my readiness is so low when my HRV is so high?

If you've take a couple of rest days off it can be a "false positive" and actually be indicating your recovery, otherwise big swings up or down are indicators that you need a rest.  

This is explained in some depth in the book if you're interested in the science behind the reasoning. (Unfortunately I lost mine after my last move so I can't look it up)

Higher HRV isn't better.  Normal HRV (for you) is better.  Higher means parasympathetic dominant (long-term strategies).  Lower means sympathetic dominant (short-term strategies).

According to the book, higher HRV in response to training means you've shifted into long-term adaptations to stress.  A low HRV in contrast is associated with short-term adaptations (such as increased hormonal output).  Any of it has to be interpreted in context.