I am still breaking my head on this, but some colour schemes look great when you see them presented in a brand guideline, but look dull/boring in PowerPoint. More and more I think that this is because what PowerPoint is: basic slide compositions and boring/neutral Arial/Calibri fonts (especially to keep things readable on mobile devices).

• Colours that come out poorly: earthy tones: brown, olive, curry, faded red, faded blue
• Colours that come out great: bright and fresh purple/red, pink/blue, mint green, used as accent colours in compositions that are dominated by grey shades and big black contrasting typography.

One of the nice things about design is that you cannot always explain/rationalise why something "just is not right".

Art: Peter Paul Rubens, Rainbow Landscape, 1636