|MadSci Network: Botany|
Before you take the principal's word for it that all pollen is green, you should suggest your students look at pollen of as many species as possible and see if that is true. This is a good time of year to look at flowers because there are many outdoor bedding plants blooming. You might also look at greenhouse plants, cut flowers in a florist shop, houseplants such as African violets, weeds, grasses, and vegetables like corn, tomatoes, beans, etc. If you can't see live plants, you can look at photos in books or on websites. If after making observations, you find that all pollen is green, then you can consider reasons why. One obvious reason is that plant tissue is often green because it contains chlorophyll. A quick search on google.com located a page that says corn pollen can be yellow or white so that seems to undermine the principal's hypothesis. An encyclopedia.com page says pollen is usually yellow, but sometimes brown, purple, red or white. References Yellow and White Pollen in Corn Pollen Color
Try the links in the MadSci Library for more information on Botany.