I am learning Spanish with Collins Easy Learning Spanish. It gives the follow example:
It is not very good = no es muy bueno
It’s very good = está muy bueno
Why does only one example use está to describe the quality of being good? Is there a rule that applies to using está for discussing good qualities?
That would be an incomplete example. As you guess you can use both ser/estar.
It is not very good
If we refer to, for example a deal, we would use ser:
(el trato) no es muy bueno
If we refer to the condition of something, we would use estar:
¿Cómo está el coche? (meaning what is the condition of the car?)
No está muy bien.
¿Cómo está la cena? (meaning how's dinner tonight?)
(No) está muy buena.
And then you have also a problem because good can be translated both as bien or bueno. So "it is (not) very good" could be translated as
So basically I think that there could be a lot of options and example did not cover all cases (probably I didn't either in my explanation...). It is not that you can only apply one verb for positive or negative, it is just that the example is not exhaustive.
Another example to explain further how both ser and estar can be used for good and bad.
El pastel de chocolate es muy bueno
In general, (all) chocolate cake is good.
El pastel de chocolate está muy bueno.
This particular chocolate cake is good.
So you may use either verb, depending on the meaning you want to give your sentence.
