In the Image Map Shader you can adjust the image via the parameters under the Colour tab. For example, checking the Multiply RBG checkbox and setting the red value to 0.5 will reduce the red value by half.
It looks like you're using the Image Map Shaders to define the final density of the Global Cloud layer. If you're unhappy with the rendered look of the cloud layer itself, you'll want to adjust the cloud layer parameters, such as those found under the Lighting tab.
There is a Density Fractal node providing the density to the cloud layer. You might also try adjusting its contrast value under the node's Density tab.