I agree with Cure, you really *should* credit whoever it was that made the palette. An obvious example is Arne's palette. We all know it exists, but if I made something with it and I didn't mention that it was his palette, it could be seen as me passing off that palette as my own creation.
Even if you are using a well known palette (Arne's doesn't count, I'm sure most people outside of pixelation haven't heard of it.),such as CGA, then you probably should still mention that you are using the CGA palette. Not stating where your palette is from implies that it's yours and is more or less taking credit for something that isn't your own. I have seen a lot of images where people have been praised for their colour choices when they are using a palette they didn't create.
Having said that, if you see an image consisting of all pure colours/well known colours (ff0000, 00ff00, 0000ff, 000000 etc...) then you probably don't need to cite anything, since they are all fairly obvious colours. It would be rediculous if I decided to make a 2-colour black and white piece and then cite someone who also used pure black and white.
What Ryumaru is saying seems like a bit of a different thing entirely. One colour is one colour, no one is ever going to make a piece of art with only one colour, well they could, but it would suck! As soon as you start having more colours then it becomes less and less likely that someone else could pick the exact same colours as you purely by chance. Think of it like this - if you just place a single pixel on a screen, then obviously you shouldn't need to be credited anytime someone else decides to place a single pixel on a screen. But as soon as you start adding more pixels to form a complete scene and someone decides they want to use your lineart for that piece, then you probably would want to be credited for it.
Basically, if a piece uses a distinct set of colours and you decide to use those exact colours then it's probably best to just credit the person whose colours you're using
