You can't mask or overlay a sprite like that. You will have to modify the shader to do it.
Also, colour is a property - you can't add to it but something like this will work:
oObject.GetComponent<tk2dSprite>().color = oObject.GetComponent<tk2dSprite>().color + new Color32(0,0,60,255);
Also, the range of rgb components in Color is 0..1, not 0..255. If you want 0..255 range, use Color32.