Clicking on More Colors from the Fill menu will bring up a color wheel so you can pick any custom color.
Click and move your mouse in the vertical column on the right hand side of the dialog. You'll see a white band that lets you move it up and down along a spectrum. This selects the color's hue.
Select a color from the spectrum.
Next, move and click your mouse in the square panel to the left. You'll see a black dot appear. This selects the color's shade.
Alternatively, you can also enter an RGB or hex value at the bottom of the dialog.