The only "staple" colours that tend to always be in the Mulberry collection are the oak, chocolate & black bags.
I believe Graphite is just a "seasonal" colour .. though I have no idea if Mulberry will be bringing anything out in this colour next year or not, sorry.
Is there something in particular that you have your eye on?
I have a small Antony in graphite pebbled leather and it's lovely. The colour is kind of shimmery grey and the leather itself is so soft and squishy. Oh, and it's brilliant in the rain, too!
I saw KLP's Graphite Ant at Bicester and was extremely envious! it's very very nice to look at, slightly edgy but not enough to stop it becoming a classic.
I have just bought a graphite bayswater from Shepton Mallet but can't tell you much about it as i'm not allowed to take it out of the bag untill Christmas! I think it's a beautiful colour though, will go with almost anything.