I googled this and this does not exist here yet, so
Why is the Ethereum paper called the "yellow paper"?
The Ethereum paper is the White Paper, originally published by Vitalik towards the end of 2013. (About a month later, in early 2014, Vitalik announced the start of the project.)
Other colours of paper are presumably plays on the fact that the colour white was already taken.
The Yellow Paper (a technical specification) was written later by Gavin Wood (at least by April 2014 - unsure of the exact date). If you print out the PDF, you'll see it's on a yellow background.
The Mauve Paper was published in September 2016 by Vitalik. Again, the background is mauve (which is a pink/purple colour).
The Polkadot Paper was published in November 2016 by Gavin Wood. The background of the document is pink with white dots.
Other projects have continued this trend. For example, the Melonport white paper is actually their Green Paper...