I've gradually realised that texture is almost as important as colour. So the infamous Crisp White Shirt, for example, drains my complexion but a white T-shirt or cotton jumper is fine. Also, a skirt/trousers with a top in the same texture can look a bit dull, but if you mix it up with a chunky jumper or silk top it's so much better. So I now aim for a mixture of rough and smooth. I find that the older I get, the more important texture is to me anyway – I'll buy things almost entirely for the way they feel sometimes.
The shortest shortcut to frumpy for me is getting the neckline wrong - it needs to be a boat neck, or a slightly dipped round neck, not a standard round neck like in a classic T shirt. I spent decades thinking I SHOULD look all right in various necklines and now I've accepted that I don't and I avoid them and feel much better for it.
There's a whole range of colours that look much classier with white than with black - camel, grey, bright pastels. So I've invested in a (washable) white skirt and (non-skinny) white jeans. After years of the safety of black it feels quite liberating (I've led a very sheltered life, as you can probably tell).
And finally - only buy something if you love it so much you want to walk out of the shop wearing it. Otherwise you risk buying it because it's something you ought to like rather than something you really like.
That is the sum total of my six decades of fashion wisdom. Other than that clothes are slippery, duplicitous bastards that can look perfectly fine and flattering when you get dressed in the morning but all kinds of hideous wrong when you're out and about and catch sight of yourself in a mirror unexpectedly.