Go to a museum such as the Metropolitan Museum of Art and look at celadon ceramics. Korea claims to have perfected the art of celadon glazing, and many fine examples dating from the 10th Century forward are in museums.
Check on art galleries such as the Clay Art Guild of the Hamptons to see if they have any celadon pottery on display.
Use an Internet search engine to find photos of celadon pottery. The colors may not be as accurate as viewing the pieces in real life, but this could show you some of the variations.
Go to a local library and consult books on ceramics. Again, photography may not show some of the subtle colors, but the books may provide additional information on where the actual objects can be seen.
Find a pottery studio and make some celadon-glazed pottery to see the colors and shades that can result from this technique.
Pick up color swatches from a fabric or paint store to see what commercial colors called "celadon" are available.