So this could possibly be way out of whack, but when she says:
"Side by side they both get bright
Together they both get gray"
It makes me think of a relationship in the sense that the two people complement each other so well and as LoserNo1 said about complementary colours, the two people offset each other and seem to grow brighter, or happier rather when they are side by side. And then with the second part "Together they both go gray", the two people so happy and in love grow old together. I don't know if that makes sense to anybody else but it's another way to look at it, even if it doesn't fit with the meaning of the song as a whole.
So this could possibly be way out of whack, but when she says:
"Side by side they both get bright Together they both get gray"
It makes me think of a relationship in the sense that the two people complement each other so well and as LoserNo1 said about complementary colours, the two people offset each other and seem to grow brighter, or happier rather when they are side by side. And then with the second part "Together they both go gray", the two people so happy and in love grow old together. I don't know if that makes sense to anybody else but it's another way to look at it, even if it doesn't fit with the meaning of the song as a whole.