While hundreds of new hair and beauty products enter the market every year: sometimes the oldest solutions are still the best.

This sentence:

While hundreds of new hair and beauty products enter the market every year: sometimes the oldest solutions are still the best.

Is incorrect.

Explanation:

The reasons this sentence is incorrect are two. First of all the colon is used incorrectly. Because it is used to provide specific information that falls into a category that is produced by the sentence before the colons. However, in our case we don't only receive an example of that category's elements. But an explanation. And explanations shouldn't be introduced by colons. They should be followed by them, making it very clear the following elements are part of the category presented.

the rigid outer part of the earth, consisting of the crust and upper mantle is the lithosphere.     answer   crust and outer part of the mantle.