How can I recreate this Material and flat Colors? I really like the flat colors under the Materials viewport in Cycles render  

How can I recreate this for use in a game engine?
Do you mean something like this?



It can be easily done by simply linking color node straight to output, bypassing any kind of shaders



But I don't know how it'll export for a game, you can always use Emission shader on strength 1, which doesn't modify visual appearance at all