1. Default color: The current blue color has #22a1c4 color code. Look for this in style.css file and replace it with color code of your choice. To do this quickly, you can copy whole style.css into a notepad file and then use replace feature to replace color code in the whole file in one go.
2. I understand you want to display “trending articles” as seen in this demo – http://demo.simpleprothemes.com/flex/blog/ – For this you need to add “Flex Featured Posts” widget to the “Home Middle” widget area. See steps here – http://www.simpleprothemes.com/configure-blog-layout-flex/
3. When you add “Flex Featured Posts” widget in specific widget area – you will get option to select image size that you want to display.
4. For social icons, install simple social icons plugin – https://wordpress.org/plugins/simple-social-icons/ – then use simple social icons widget in any footer widgets section of your choice. Also, read this – http://www.simpleprothemes.com/configure-flex-footer-area/
I recommend you give Flex help guide a quick read to get started faster – http://www.simpleprothemes.com/flex-help/