Best use of Keywords for E-commerce website

Hello everyone, i would like to ask some questions about the best use of keywords and main keywords for the E-commerce website and particulary the product pages.

1. If you have many similar products to sell, i hear and see that every store make a new product and not a multiple product for each color for example : 4 case in leather for iPhone 6 in 4 differents colors.

So you have to make 4 differents products pages with similar title and only change the color or with different title?

like that :
Leather case for iPhone 6 Black
Leather case for iPhone 6 Blue
Leather case for iPhone 6 Red
Leather case for iPhone 6 Pink

For those 4 similar products, u want for example the main keyword sentance "Leather case for iPhone 6" and you are using woocommerce with yoast SEO, the plugin says its bad to use the same keyword again and again but come on for an E-commerce its inevitable, so is it really bad for SEO ? What is the best use ?

2. I hear that it was good to add many content for every pages so i guess also for the product pages. Is it good to make a different text for every similar product ? Is it just super bad to make the same text in two different page and google see that as "Copy content" ?

3. Do u know an alternative of google keyword planner (The results are approximativ and only available if you start a campagn.)

4. Is it really good to make all your website on keyword longue tail if you don't have a big budget SEO or not at all or is it better to stay on popular keyword or a mixt of both?

5. Is it good to turn on "Ask search engine to don't index this website" in wordpress if the website isnt finish yet and already online (...) ?
It's all for now, thank you very much