[Lonthego]

Supperman

Just a few corrections.

"I think Okinawa sea coast is the best, when I talk about the color of them"

Instead of "When I talk about the color of them" it sounds more natural to say "in terms of color" or "when it comes to color"

For example "I think the sea coast of Okinawa is the best in terms of color" or "I think the sea coast of Okinawa is the best when it comes to color"
You could also reverse the order and say "In terms of color I think the sea coast of Okinawa is the best"

Also, at least in the US, in spoken conversation people also use the suffix "-wise"
"I think the sea coast of Okinawa is the best color-wise"
It is generally not accepted in formal writing though.

Next,

"Things becomes different when it comes to beach coast" -> "Things become different when it comes to beach coast"

Just a final note, some of your possessives, while often grammatically correct, sound too Japanese, such as
"I don't know why the Okinawa's sand color is so white."
Here most native speakers of English would say "I don't know why the color of Okinawa's sand is so white" or just "I don't know why Okinawa's sand is so white"

Same with "Yet, Okinawa's sea color is quite different"
It would be better to say "The color of Okinawa's sea"
This is also partly because "sea color" and "sand color" themselves are not often used.

Just something to keep in mind. Since you are already very good at grammar and have a wide range of vocabulary all it really comes down to is idiomatic usage.