Colours
How do you tell them apart? Which one wears which colour (red/blue/green)?
The short answer is that Huey wears red, Dewey blue, and Louie green, but that this has not always been the case.
In the old cartoons HD&L wore different colours in different cartoons, and sometimes two or all three of them wore the same colour, very often red. In their very first cartoon Huey wore a green shirt and cap, Dewey orange and Louie red.
So this red-blue-green thing was invented in the comics, where their shirts are black but their caps have different colours. Earlier the colourists weren't consistent about it though, so not only do they wear different colours in different stories, but also they seem to switch caps between panels!
The current official colour scheme was first used in the modern TV animation, i.e., in DuckTales, and later adopted by Gladstone and Disney Comics most of the time.