I think most people would agree that actual colours are not super-important. Most people mix some colours and for the most part all you need are good strong primaries as a mix-base. If you desperately need a certain colour you can dig around minis paints until you find a good one, mix something up, or in the worst-case scenario actually get into an art store and buy real high-grade artist's acrylic.
But those washes are pretty unique in their effect, because it's not just the colour but also flow and coverage. There's several variables that come into play at once and the GW ink washes really were one thing where all those elements just seemed to work perfectly.