It can be extremely difficult to match new concrete or stamped concrete to existing concrete of any kind. Two suggestions. Either strip,re-stain, and seal your existing patio simultaneously with the new concrete patio to blend the new and the old and try to match them up as closely as possible. Otherwise you might consider using some kind of material to create a boarder or separation between the two patios. This will break up the patios a bit and you could create contrast between the old patio and the new patio by installing a completely different type of concrete around the pool. This may give the effect of creating a separate space between the new pool patio and the existing stamped concrete deck.