I've found with the help of TherioN from NUIGroup forums a way to do it. It is possible to add a MTRectangle with the fill color and then add the SceneTexture of the drawing example to that rectangle. I leav the piece of code as reference:
final MTSceneTexture sceneTexture = new MTSceneTexture(mtApplication,0, 0, mtApplication.width, mtApplication.height, drawingScene);
sceneTexture.getFbo().clear(true, 255, 255, 255, 0, true);
sceneTexture.setStrokeColor(new MTColor(155,155,155));
//Background
MTRectangle background = new MTRectangle(0,0,mtApplication.width, mtApplication.height , mtApplication);
background.setFillColor(new MTColor(255,244,150,255));
//Add the scene texture as a child of the background rectangle so the scene texture is drawn in front
background.addChild(sceneTexture);
frame.addChild(background);