When you doubleclick on one of the boxes, at the bottom of your toolbar,
it pulls up the color picker box, shown below.
You'll notice three boxes with color in them - a large one on the left, a tall narrow one on the center, and a small one on the top.
Click anywhere inside the large box to select a range of colors. That gives a narrower range in the tall narrow box to choose from. Then click again inside the large box to choose the color that you want. You can also move your mouse around on the box to get a wider range of colors to choose from. The color that you choose shows up in the third box at the top.
Move your mouse around inside either of the first two boxes to choose a color, then click on a spot to select a specific color, then click ok to close the box. The color you select appears at the top of the third box.
On the right side you'll also see the RGB and CMYK settings that identify the color that you have selected. You can just type these settings in to get this color.