What you want to do is first find your eye colour. Then, what is the colour dirrectly opposite your eye colour?
That's the colour eyeshadow to use.
So, if you have blue eyes, golden orange tones will make your eyes pop.
Green eyes, red or plum tones will make them pop.
Hazel eyes (the yellow part), purple tones will make them pop.
Brown eyes, it's great to add some colour to the eyes like a blue or a green to add that pop factor.
Now, colours on the eyes can seem so.....festival-y. I hear ya.
For day to day, or, even a nighttime event, some colours can sound not subtle and beautiful.
But, believe me, it truly can be added in subtly and you don't even notice that's 'colourful' because your eyes are the focus.
Also, the trick is just adding it in a little bit, such as through the crease and blended into a shimmer or a brown. This is what keeps it subtle and not overboard. I'll show some examples below of different looks I've done using colour in a wearable way.