Add a legend with explanations for the used colors and remove inconsistencies

[Zodiac1978]

I am not sure what the current color code means. What are the definitions which color is used in which case?

In the "Tested" column "5.3" is blue, but "5.3.0" green. 5.2.4 and below is red. Maybe the last 3 major versions could be marked orange instead of red, following the warning in the plugin directory and the inconsistency with 5.3/5.3.0 could be solved.

If the "installed version" is "6.3.8 (6.3.9 available)" it is marked green although it is outdated (Newsletter plugin). On the other side "3.2.10 (3.3.3 available)" is marked red (All-in-one-SEO) which would be the expected behaviour.

Sometimes the version is blue and not green, although no update is available. Why not green too in this case ...?

[roytanck]

The idea is to have a traffic light type system (green, orange, red) eventually, but I found that a lot of situations were "neutral" or hard to interpret, so I initially used blue instead of orange. 1.1 uses the default text color instead of blue.

Interpreting the data from the wp.org's API and applying the correct color definitely needs work.

[roytanck]

I fixed an incorrect use of the version_compare function. Version comparison should be more consistent now.

• Exact version match = green
• Major version match = neutral / grey
• Earlier major version = red

For percentage (rating column)

• 90% or more = green
• 70-90% = neutral / grey
• Under 70% = red

For time differences (last update column)

• Under 90 days = green
• 90 days to one year = neutral / grey
• More than 1yr = red