Hello all, My photos come back with the number printed on the back already by the developer. It is very light but readable. On writing on the back of present day photos, I use an extra fine sharpie. They come in various colors and I use the lightest color that I feel is comfortable enough for me to read. At present I am using green. I don't use ink pens because they have a funny odor to them and I think that whatever is causing this odor is probably not good for the photos. I've also found that older photos sometimes have the ink bleeding through. On older photos, I've gone to an art store and purchased the graphite pencils that drafters used to use. Some are extremely soft so I do not need to use a lot of pressure to write. Hope this helps :) Kathy L