If, at your butt joints, you applied tape/epoxy and later coated the rest of your hull/deck you could have differences in color due to the action of light on the uncoated wood as apposed to the coated wood. This is why it is good to try and precoat in some cases or at least try to coat the entire sheet when applying the tape to the butt joint. There are some solutions, ie. try to mix up a colored epoxy and artistically apply it in the lighter areas while trying to blend the whole thing together....BUT...it probably isn't worth the effort. As has been said, this imperfection will become much less important as time and scratches come along. NOBODY builds a perfect boat. You will probably be your harshest critic...but...even you will mellow out and learn to love your imperfect little kayak. Your next boat will probably be even nicer.