Sure, if there's sufficient contrast within your flat design, I'm fine with that. Lots of good examples of this in 'material design' where buttons are easily re

Sure, if there's sufficient contrast within your flat design, I'm fine with that. Lots of good examples of this in 'material design' where buttons are easily recognised, with nice colourful backgrounds. When buttons get wishy-washy grey in some attempt to "blend in", then we have bad flat design.