Let's start with a more obvious one: red is a passionate, attention-grabbing color. It's appetitive; relative to the other colors, it's more likely to stimulate your hunger (whereas blue is more likely to suppress it). It's the color of blood, and so we think of literal blood—all the scariness and fun that entails—but also lifeblood, the passion that drives creativity and personality. Red means danger, but in the best possible way.
Yellow is bright, warm, and sunny, but it's a tricky color to get away with; it doesn't match with everything, including most personalities. Yellow is a terrible color for text and writing most of the time (see above); fortunately, it does great to highlight and it pairs fabulously with both warm and cold. Yellow doesn't always fit—but where it fits, it excels.