I recently worked with a talented adult student who is beginning to explore watercolor, after having done some beautiful charcoal drawings and a first time acrylic painting. Although watercolor is often described as an "unforgiving" medium, we're both enjoying its transparency and sometimes unpredictability. What better choice of medium is there to depict the transparency of colored glass than transparent watercolor?
It's not necessary to purchase expensive materials to get excellent results with watercolors. Check out the Yarka set of twelve colors. It's a good quality, inexpensive set which is appropriate for both children and adults. Using decent watercolor paper will enhance your results too.