I have been searching though the Brooks website looking at their saddles and I spotted these bad boys.
|Brooks B17 Standard "Ochre"|
|Brooks B17 Standard "Black"|
Also saw on the Brooks website it mentions "custom colour service" regarding saddles so maybe look into that some more. If the Ochre colour isn't a good match for Brompton's yellow maybe ask for something closer to it or maybe even be completely bonkers and get a black and yellow striped Brooks saddle?
I have to say I'm abit taken with the Ochre coloured one but been warned that the colour doesn't always stay as good after time. Dirty, grease, grime, etc would probably make it darker in time, just have to look after it.