You can copy (sample) a color from any bitmap image by first importing the image into the 3D View as an image plane, and then using the Color Picker.
The cursor changes to an eyedropper icon indicating the Color Picker is active.
The color is sampled and loaded into the color channel for the paint brushes. The next time you paint using that brush the color you copied is used.
The Color Picker tool remains active until you select another paint or sculpt tool.