BMF is great for masking complicated break lines or for small irregular areas.
Not only is it much more pliable than masking tape for working into tight corners using a toothpick or cocktail stick, but you can get an even neater result by applying one or more pieces of BMF roughly straddling the line to be masked then trimming very precisely to the line you want using a craft blade (with very light pressure so as not to score the first colour). Gently lift away the unwanted areas of foil with tweezers, then burnish the foil along the edge of the masking line with a cocktail stick, a cotton bud or even your finger, stroking towards the cut edge of the foil rather than away from it, so as not to lift the edge. Having used relatively expensive BMF to mask the immediate break line between the two colours, the rest of the model can be masked with cheap masking tape, overlapping it firmly on to the BMF, before spraying.When spraying, try to direct the spray away from the edge of the masking line as much as possible, rather than directly towards it, to minimise the risk of the spray forcing its way under the foil line.I see John Quilter suggesting Scotch tape (which we call Sellotape in the UK). Maybe he has managed to make that work but I find normal adhesive tape quite inflexible, and too "grabby", and it adheres quite strongly so personally I would be afraid of it pulling off paint - unless there is a low-tack variety sold in the U.S.?