There’s many web based color picker tools out there, but when you want to learn as much as possible it’s hard to go wrong with Color Hexa. To quote the description:
ColorHexa.com is a free color tool providing information about any color. Just type any color values (view full list here) in the search field and ColorHexa will offer a detailed description and automatically convert it to its equivalent value in Hexadecimal, Binary, RGB, CMYK, HSL, HSV, CIE-Lab, Hunter-Lab, CIE-Luv, CIE-LCH, XYZ and xyY.