I guess I am specifically referring to red onions, (but this goes for all onions) are there more nutrients concentrated in the outer (red) layers, compared to the inner core and green parts?
Generally, there are higher levels of micronutrients in the peel/skin of plants compared to the inside/flesh. Also, the deeper the color generally the more micronutrients. Think the peel of an apple compared to the white flesh. The peel is more colorful and consequently has higher levels of micros compared to the flesh. There are probably exceptions, but in general yes the outer layers have more nutrients.
Doesn’t mean you should throw out any part of the plant; just try to save as many outer layers when you peel your onion and eat the flesh of plants if it’s edible. This will help get the most micros from your diet.