Adobe Color Wheel

Adobe Color Wheel is a site I use regularly when choosing color schemes for themes/user groups/etc. It allows you to choose colors on a wheel, and gives color suggestions based on a color rule (monochromatic, triad, complementary, compound, or shades) and will alter surrounding colors when you alter one, to keep them within the color rule. It REALLY helps to match colors.

You can also create a custom pallet, which will let you change colors without altering others. Additionally, you can save these color pallets, which has been useful for me so I don't have to go hunting for color codes.

Hopefully this is helpful!