@distel hm, the page Wikipedia's quoting is cautioning against mixing inks willy-nilly because it "sometime causes problems with a reaction between the various chemicals etc". https://web.archive.org/web/20140407091022/http://www.marcuslink.com/pens/ink/index.html
Wikipedia quotes it but talks about ph changes changing the color in unexpected ways and particles going out of solution when the solvent concentration changes