New young grass is normally lighter green.
HOWEVER...if the new seed has some ryegrass in the mixture, or if it is a seed that produces a lighter green grass, you may have some persistent spots that are a different color.
So true, it is hard to find out exactly what grass seed was used to grow the original sod; and it is hard to find out what would match it. And maybe you don't want the same seed if it got diseased. And Kentucky bluegrass is difficult to get much seed "take" on top of old bluegrass.
Try the seed superstore for a premium, dark green, disease-resistant type.
If you have no other choice. Buy rolls of sod and just resod the spots. Quick. Easy.