On the interior surface of the concrete, if there are any stain and discoloration then it is very much possible to fix it. All you have to do is to rub the surface with a scrubber dipped in hot water and repeat this process until it removes the stain completely. In case, if you do not want to work that hard then you can give your concrete surface with an acid wash treatment and this will penetrate through the surface and remove the residue of the stain. But if still, these methods are not helping you in repairing the discoloration of the concrete then you have to use the color that will blend with the other area of the surface.