I wouldn't worry about it too much. All the garage will do is look at your VIN plate, match it to a 3 symbol code, and flick through a kinda directory to find how many mils of each colour they need to mix to get your colour. If they've been in the business long enough, they'll know what they're doing. Just make sure that the garage are willing to supply a warranty on their paint repairs, otherwise steer well clear.
However, I suppose it wouldn't do any harm if you contacted Lexus and asked them about the supply of paint.
BTW, don't worry about the cleaning - when you see the finished result with black, you'll want to keep it clean all the time anyway ;)