What is the benefit/advantage of using the flat UI colors/design?

First of all, in my opinion, there is nothing like "Flat Colors". You can use almost any color, as long as it fits well to the design needs and plays nicely with the rest of the layout. I love Flat Design and am very thankful it became a "trend" in nowadays design. Here are some reasons why: Design is all about process. You don't only design to make something beautiful, but first and foremost to reach business and user needs. Flat is very simple in terms of form, and that helps you focus on the usability of your site/app. If you follow few rules, your design will look tidy... ...but it does not mean that it is easy to make a great design. As you have observed - many sites seem complicated and flashy. You really need to plan your design before digging into Photoshop. It is also light - as you do not use Photoshop effects, you do not need to insert (many) images into you code. Responsive Web Design - flat makes it a lot easier to make sites responsive, as you do not need to scale images, effects and so on. You just have vector shapes and colors. It helps user focus more on the content. Flat makes leading the user through site easier. Design is now very clean so the only emphasis you need to use are fonts, colors and shapes. It is not as easy as it looks and sounds. You really need to put a lot of effort, time and planning into your work to achieve great design. You can also easily fell into trap of creating layouts looking "almost-the-same", so it is important to remember about leaving some touch of brand and diversity in your project (like I have done with background elements and shapes in https://www.behance.net/gallery/12064291/GMDP-Services-website-design). Flat also doesn't mean to me not using gradients at all. For me it is making everything as simple as it can be, but not simpler and avoiding "old school" Photoshop effects (like Bevel&Emboss for example).