Absent any other context, I would have to assume that the speaker is suggesting that the listener is likely to vomit all over the front of a piece of clothing they are wearing. However, that's a guess. Please give us the surrounding context and dialogue.
Here is the context:
Well, I may send a telegram if I can work a few days off at Athens. If I go on like this you won’t want to meet me. You probably don’t now as it is. When I got your letter I knew you’d just written it because you were bored out there. Isn’t it awful I still have to get boozed to write to you. It’s raining, I’ve got the fire on it’s so bloody cold. It’s dusk, it’s gray it’s so bloody miserable. The wallpaper’s muave or is it mauve hell with green plums. You’d be sick all down it.
In addition, there is another phrase here that is unclear to me: 'if I can work a few days off.' Does it mean: 'if I can manage to have a few days off'?
In addition, there is another phrase here that is unclear to me: 'if I can work a few days off.' Does it mean: 'if I can manage to [STRIKE]have[/STRIKE] get a few days off'?
And, apparently, the wallpaper contained arsenic in the colours, so he may have been more correct about how damaging to his health it was than he knew. This, of course, may be incorrect, but it's too good to find out it was wrong.