The color or something can effect only some parts of artwork, color effects the mood and some other things where as if the painting only has color and doesn't have value you cannot tell where the conturses of the shape are,and you cannot tell things like if it drops off anywhere ,or even if it has a curve or indent in the object.Value gives you depth in the painting, it helps you sort out where the light source is coming from. When used properly value can give an almost 3D appearance to a shape were as other wise without value you couldn't, different values can also help depict any of those kinds of highlights from the light source.
