do I need to create 2 listings for each SKU? (which means for me in fact 8 listings as I have 4 color variations?) –YES,
should I send part of my inventory to Amazon and part (the rest) to 3PL?? – YES – send the maximum you can into FBA and the considering either hold a proportion or sending the remainder to your 3PL. NOTE, some sellers who are based in the USA can hold stock at home and send small quantities into FBA and ‘drip-feed them’ if they need to do too. This is instead of sending ALL remaining stock to a 3PL and having to pay high transportation costs which are charged by your 3PL. (sometimes you can ship locally cheaper in SMALL quantities than they charge)
Should I use FBM listing for the launch only? Yes,/No – yes you will use it to launch but it will be valuable for you to keep this listing in stock in case your FBA goes out of stock due to shipping and FBA clearing delays.