Otto was endearing, creative, sweet, lovely even.
Even if he got on Gibson's nerves every now and then, he knew he didn't truly mean to. He just wanted to be helpful and sweet. He couldn't put Otto at fault for such a thing, even if he didn't always get Gibson's social cues. The mechanic was intelligent in his own way. He understood things that Gibson couldn't, even if Otto couldn't understand science as well as he did. Otto got him to smile at times when he was frustrated.
He was the sunshine and smile Gibson needed.
Unfortunately, there was so much he could say and he couldn't do so with one color. There was nothing green that had everything he felt for him. Carnations, however, had everything he needed, even if it was multiple colors. Greens, oranges, reds, whites, they were perfect for what he wished to say. They weren't much in terms of quantity, but they said plenty of words on their own.
Gibson laid the bouquet of carnations close to Otto as he worked and left. Otto merely looked back before he picked up the flowers with the note attached to them.
Common meanings: Beauty, distinction, fascination, and love.
Green: Gentleness.
Mixed colors: Health and energy.
Orange: Happiness, creativity, and enthusiasm.
Red: Admiration and passion.
White: Adoration, endearment, innocence, purity, gratitude, and respect.