PowerPoint wasn't invented in the Middle-Ages, when knights rushed around battlefields in full armor. But the slide decks of today share a common characteristic with those knights of old. Both need to be able to clearly distinguish symbols from a distance. We may not have mud and dust, but many projectors aren't all that great at contrast.
To help medieval knights tell friend from foe, they emblazoned their symbols on themselves, but these symbols needed to be clear. As a result when you're creating heraldric arms, you need to follow the Rule of Tincture - and it's a handy rule for your PowerPoint too.
The colors you use in heraldry are divided into two classes: metals and colors. In the basic heraldric system there are two metals: white (silver) and yellow (gold). Everything else is a color. The rule of tincture says that you cannot put a metal on a metal, or a color on a color. Applying that now, it means that if you have a blue (color) square, any text you put on it should be a metal (white or yellow).
As I've stated it above, the rule is simplified for heraldry, let alone slides. But I've found it a useful guideline to keep my slides easy to read.