The colours indicate where to find good spots with high wind probability at what time of the year. A value of e.g. "60-80%" means, that in this country there is at least one good kitesurf spot with some infrastructure (a kite center with rescue) where - statistically spoken - you'll get on your board on 3 to 4 out of 5 days - without having to “chase the wind” on different spots.
Such a "windy day" is defined as at least 3 hours in a row during daytime with at least 12 knots (4 Bft). Please be aware that all these values are only estimations. They are carefully derived from wind model data from the last 5 to 10 years combined with hundreds of trip reports - but as wind patterns may change from year to year, there's never a guarantee!
And please keep in mind that these "windy days statistics" doesn't say much about the typical wind strengh!