Color Selector...

Color Selector lets you interactively select a color for Mathematica graphics. (Windows and Macintosh only.)

When you have chosen a color, click OK to paste the selected color at the text insertion point, cell insertion point, or selection within a cell, in the form of a Mathematica RGBColor[red, green, blue] function. In this way you can specify the color of a graphical object without having to figure out the actual RGB values for the color yourself.