Shading area under curves in the android charting engine… here is a solution to the problem.
Shading area under curves in the android charting engine
How to color the area under the curve in the worm chart line of achartengine for Android. 
Solution
This should be simple:
FillOutsideLine fill = new FillOutsideLine(FillOutsideLine.Type.BOUNDS_ALL);
fill.setColor(Color.GREEN);
seriesRenderer.addFillOutsideLine(fill);