It appears that the colours are not on a fixed scale. The season averages and individual episodes are coloured using different ranges.
It ranges from lowest to highest regardless of the value. Like conditional formatting in Excel does if you don't specify the scale.
The seasons average ratings range from 6.1-8.4, so it goes red to green in the span of 2.3 points.
The episodes range from 3.9 to 9.3, so it goes red to green in the span of 5.4 points.
The full IMDb ratings range from 1-10. This should have been used as a basis for the colouring instead. The overall average on IMDb is somewhere around 7, so it would be fine to skew the colours so the middle/yellow was at 7, but it should be able to represent any possible ratings.
Should probably also be acknowledged that the sample size is not going to be the same.
You're going to get a bunch of people piling in to highly rate the early episodes that they remember watching when they were kids, but a significantly lower number are going to be voting on the episodes that came later.
Really the whole premise of trying to compare and contrast the seasons for such a long running show that existed before IMDb even started is flawed on many levels.
Yes, it has been observed that IMDb sometimes get a lot of new "single post users" putting in 10/10 ratings on Disney movies that otherwise scored badly.