Most of the time, the color of the mucus coming from a child's nose doesn't mean anything. In fact, most colds start out with a clear runny nose and end with a yellow or green runny nose. Colds are caused by viruses, and the color comes from your immune system attacking and getting rid of the virus. If your child's runny nose changes from clear to colorful you should take it as a sign that he should be getting better soon.
When is a cold not just a cold? When is it something else, like a sinus infection? Sometimes, even doctors can have a hard time knowing for sure, but there are some general guidelines. First of all, most colds start to get better after five to seven days. A sinus infection is defined as nasal congestion lasting 10 days or more without improvement. Those two words "without improvement" are important. If your child is still a little congested on day 10 but overall is feeling better, then it's probably not a sinus infection. Another indicator would be a sudden change in symptoms, especially a new fever. Many kids will have a fever on the first day or two of a cold. The fever then goes away and the cough and congestion last five to seven days. However, if your child has been sick for a few days and then suddenly starts having fever and feeling worse, then that could be a sign of something different, such as a sinus infection, an ear infection or pneumonia.
Following the guidelines above should help parents avoid unnecessary visits to the pediatrician. However, as we all know, kids don't always follow the rules. Parents should always follow their gut. If you are worried, then by all means take your child to the doctor. I think the most important job of a pediatrician is to determine which kids are truly sick and which kids will get better with time. I never expect a worried mother to make that determination on her own.