Kettle caramelization can add quite a lot of colour. Actually, in some of my recipes I follow Randy Mosher's suggestion of taking a small part of the first wort and boiling it separately until reduced and caramelised. Beer in which I have done this can be quite darker than the colour calculated just from the ingredients.

So I propose that along with the Morey, Daniels and Mosher's SRM formulas there is a "custom color", or even a "custom formula" for the experts, although I suspect that it would be kind of hard to give users access to the internal variables and values, specially if one thinks that the ideal formula includes boiling time, kettle caramelization and other odd things I can't think of.

Anyway, just "custom colour" would be enough for my purposes.

Umoyk