i'm considering house of colour but i dye my hair various shades of brown to cover the grey....and i have a sunbed habit just before i go on my hols.....will this affect my colours? don't want them to only suit me part of the year.
No. They will cover your hair if it is coloured, it doesn't affect the outcome, though it might affect what colour you choose to dye it in future.
Tan makes no difference, but they ask you not to use "fake" tan on the lead up to your draping so that the tones and shades are all "natural" (be it tanned or otherwise) - that's why they also cover your hair.