What does “You’re particular, for a shade.”

From Dickens, Charles. A Christmas Carol mean?

See a translation

I will break it into 2 parts.

You're particular, means to be fussy, very careful, or overly precise, in some way. For example, in one's appearance, making sure that not a single hair is out of place. In the case of your example, Marley, was being very precise with language. Interestingly, if used to describe another person, it is often used as a criticism, but if used to describe oneself, it is not. Someone could say, I am very particular about which restaurants I eat at, meaning that the person feels they have very refined tastes.

The second part, is actually derived from the expression "to a shade", and it emphasizes what it goes with, attentive to even the slightest change in colour or light (both are shades), but used figuratively. I would say this is not a common expression these days.

In Christmas Carol, Scrouge, is responding to Marley's correction of Scrouge's previous statement. He is about to say "You’re particular, to a shade.”, but in order to make a joke to himself, instead he says "You’re particular, for a shade.”, the joke, referring to the fact that Marley is a ghost, appearing as just a shade of light.