Consider the following two sentences:

1. 早晨的太阳为什么是红色的
2. 中午太阳又变白了

For the first sentence: the second 的 is because 红色 is 区别词 and the first 的 expresses alienable possession (i.e. no close possession).

But in the second sentence: there is no "的" after "中午" as I would expect. Why?

I also found the following sentence: 你还得提高教育质量. In English, I would say "quality of education" and not "quality education". Hence, I would also expect a "的".

Another difficult case is "科学模型" vs "科学的模型".

Can somebody clarify how to use "的" to express "of"?