partly because in your orange shirt you look like a better happier St. Sebastian (3)
Since he was a Roman soldier back in his day, we're guessing that St. Sebastian probably didn't have too many orange shirts hanging in his closet. In fact, most visuals depict him more like a shirtless pincushion than anything else. Still, our speaker sees this orange shirt as a visual marker of the addressee's joy and perfection. This you is even "better" than a saint. And all it took was an orange tee.
partly because of the fluorescent orange tulips around the birches (5)
This line is intriguing for a couple of reasons. First, who knew that light bulbs came in orange tulip form? We kid. Actually, this orange is a visual echo to the orange tee shirt that our speaker praises in line 3. Again, the color suggests a kind of joy, energy, and overall warmth to the scene that the speaker is describing. It's as though things take on a vibrant appearance as a result of the love he feels. Or do you think this is a case of his emotions coloring what he sees?
partly because of the secrecy our smiles take on before people and statuary (6)
Sometimes, love means that even your facial expressions become totally inscrutable to everyone but you and your honey.