As I mentioned above I have used 2.5 mm hook and Yarnart Jeans threads. It is mix of cotton and acrylic, but it is also possible to use only cotton threads.
The main secret is to use bright summer colors. In addition I have decorated hat with beads.
I would like to crochet the same kind of hat but in yellow or bright pink, I am sure it will be amazing. Yarnart Jeans has big variety of glowing colors.
Follow this link and you will find easy explanation with bunch of pictures http://crazyhands.net/beautiful-crochet-baby-flower-hat/

You are welcome to follow me on Facebook https://www.facebook.com/groups/crazyhands.net/

Instagram:
https://www.instagram.com/amurushka_amur/