My dd really responded to us focusing on colors, and we would try to "eat a rainbow" every day. We are dye-free, so we talked alot about colors that God made, (which are awesome) versus colors that people made, (which are made from chemicals) and that the colors God made are really, really important to be strong and healthy. So she'd eat some raw broccoli with her lunch, some pineapple for a snack, etc, trying to hit all the colors in the rainbow every day. We didn't always hit it (blue and purple are hard!) but I have to say, it was a fun and ongoing thing in our house, and she really, even at almost 7, gets how important it is.
Her dentist commented that she can always tell the kids who eat lots of healthy foods, and that went a long way with dd also!