We've discussed the Queen's outfits a couple of times already, and one of the common themes running through her wardrobe is that she's always brightly colored. You'll ever see her in anything drab, gray or dark outside of a funeral or an equally somber occasion; her usual manner of dress is almost neon. It's not just an extension of her personality - although she's said to have a keen sense of humor.
The Queen is aware that when she makes an official appearance somewhere, people line up to catch a glimpse of her. With crowds several rows deep, it's not always possible for would-be well-wishers to get close enough to say hello, or even see her properly. By wearing bright colors, she's deliberately trying to ensure that she stands out from the crowd, and so those who've taken the trouble to come out and see her can at least say they saw her with their own eyes, even if only from a distance.