Using specific gradient Layouts you can modify the gradient of color. Those Layouts are editable if the color database has gradients. If the color database has colors, you can't edit the gradient. In such a case the gradient is only to show the percentage.
1. Press the highlighted (cube) icon to see the options. In the menu list, you can select "Color to gradients.." or "Gradients to color...".
2. Select one of the gradient Layouts in your color library and chose the option Colors to gradients.
3. Click on the color in the gradient that you want to modify. The color picker opens and you can change the color.
4. Select the color using the color space or values and press Ok.
5. The new color will be applied correctly to the gradient. If you wish to go back to colors, chose the option Gradients to colors.