This does android

https://play.google.com/store/apps/details?id=com.pulgadas.hobbycolorconverter&hl=en

This does iphone only

http://www.appszoom.com/iphone-apps/lifestyle/model-paint-42_dlnhv.html

I use the top (android) one and it's reasonably handy but certainly not inclusive of everything, also has a scale measure converter.  The paint-42 one looks better but is only available for the evil apple empire