Come to the Outer Hebs, the vast majority here are coburni unless there's a big drift from Scandinavia. Yes they are darker above with more contrasting head patterns, also very often much more heavily marked below with the spotting merging into dark streaks, and often on a less clean white base. The darkness does vary a bit though.
Most pass through in Autumn but some winter here, but most in Ireland, mainland Scotland and Iberia. Spring passage is just starting and probably 90+% of birds most springs passing through the Outer Hebs are coburni. They should stand out in a flock of Scandinavian birds.
Cheers,
Andrew