I'd do a backup of your GT5 save game before respraying the Miura. For some reason when you buy the Miura the paint chip doesn't get added to your items, so if you respray the car and don't like it and wish to revert to the original yellow, then you wouldn't be able to do it unless you go back to your old save. I'd just play about with different colours until you're happy with it.
I resprayed my Miura with a red paint from another Lamborghini and it looks just as good in red