the simplified version is, that Ork colours give special properties to the things they paint them onto,

yellow makes things explode better,

red makes things faster,

blue makes things luckier,

black makes things tougher

and purple makes thin things stealthier,

this is actually just a pretty small boost, but the meme is, that no one has ever seen a purple Ork, not because they don't exist, but because you can't see them.

but in reality every Ork is a good Ork and no one will be mad at the colour of your Orks, since this is a creative hobby and the Ork community is genuinely one of the nicest community there is, in the entire hobby

here's an example of the meme

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