First of all, there are stacks of FAA AMEs in the UK who can test you, without having to go to the USA.
Next, how many errors did you make on Ishihara? FARs require no more than 6 errors on the 24 plate version (although the manufacturer states a maximumof 2 errors). So, if you make more than 6 errors you will also fail Ishihara for the FAA.
Alternative testing for the FAA is different to that for JAA. The UK uses the Beyne and Holmes-Wright Lantern Tests, which you have already failed. The other tests accepted are the Anomaloscope (forget it!) and the Spectrolux, presently only available in Switzerland.
The FAA uses the Farnsworth Lantern Test, which can be arranged in the UK, or a Colour Signal Lamp Test but this is a practical test at an aerodrome and must be conducted in the USA. The FAA intends to do away with these alternatives in the foreseeable future.
You will not be permitted to fly UK registered aircraft commercially with FAA licences but there are many countries (outside the EU) that do recognise FAA licences.
There is a great deal of information in these pages and whilst there may seem a lot to trawl through it may be worth your while - you'll soon be able to sort the wheat from the chaff.
HTH
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