"not sure why there isnt just black or colour ink do i need magenta"
Colour printers print by mixing the CMYK pigments - Cyan, Magenta,Yellow and Black. Some photo quality printers use two additional colours - Light Cyan and light Magenta.
Cheap printers take two cartridges: a black one and a "colour" one. The colour one actually has 3 ink tanks, one each for CMY. The disadvantage of this type is that if (say) the Cyan one runs out because you've been printing a lot of skies, you have to chuck the entire cartridge away and replace, even though the other two tanks might be full.
Better quality printers have a separate cartridge for each colour.