Not all LED's are the same brightness, when ordering you need to know that the lumen output of all the colours is the same. Also a 1W white will be brighter than a 200ma red etc etc etc

It depends on what effect you are after also - the white LED's are PURE white - anything incandescent inc neon and Flouro is white from heat or phosphour powder and as such is a yellow/white colour.
This is why the white LED's look much sharper and have a slight blue tint to them.

Putting coloured LED's into a coloured plastic will give a deeper and richer look, but not necessarily brighter.
if you put white into these then it will be brighter but the colours will not be so deep as the white will thin it out somewhat.

So long as you have the power capacity, then you could put a 1/2W coloured in somewhere against a standard 350ma white