Jonathen:

I think simplicity is beauty because of those reasons.
First, simplicity usually means clean and neat. When people see something that is neat, almost of them feel good. And beauty makes people happy or feel good. Simplicity and beauty have that thing in common. So I think simplicity include beauty.
Second, a perfect sphere with one color is better than much colorful polyhedron. Because  too colorful polyhedron sometimes looks messy. For example, you may have an experience that you failed a work in art class because you tried to make your work colorful and beautiful. But it was not more beautiful than other simple works.

Better:

I think simplicity is beauty because of the following reasons.
First, simplicity usually means clean and neat. When people see something that is neat, most of them feel good. And beauty makes people happy or feel good. Simplicity and beauty have that thing in common. So I think simplicity includes beauty.
Second, a perfect sphere with one color is better than a much colorful polyhedron. Because a too colorful polyhedron sometimes looks messy. For example, you may have an experience that you failed a work in art class because you tried to make your work colorful and beautiful. But it was not more beautiful than other simple works.

Explanations:

1. The determiner 'those' is used to mean something or someone that has already been talked about .  It's better to use the word 'following' or 'as follows' when you want to inroduce a list or description.
2.Use the article 'a' before a noun to mean one thing or person
3.A singular subject takes a singular verb.
4.The adverb 'most' is synonymous with the word 'majority' which means the biggest part of people or things

Comment:

The saying 'simplicity is beauty' states that inner beauty matters more than physical beauty. I hope you learned some insights from this expression. Well done ! ^^