State Statute
Technically the only way to be sure is to read the PA statutes on lighting, which after taking a quick look aren't that easy to navigate (very common). This is what I found in PA:
TABLE IV—LOCATION OF REQUIRED EQUIPMENT
ALL PASSENGER CARS AND MOTORCYCLES, AND MULTIPURPOSE PASSENGER VEHICLES, TRUCKS,
TRAILERS AND BUSES OF LESS THAN 80 INCHES OVERALL WIDTH
Turn signal lamps.
At or near the front—1 amber on
each side of the vertical centerline,
at the same height, and as far apart
as practicable.
On the rear—1 red or amber on each
side of the vertical centerline, at the
same height, and as far apart as
practicable.
What the statutes don't comment on is to be amber does the cover have to be amber or can just the end result be amber. If I was a betting man I'd say a judge would have a hard time justifying a ticket if the end result was the same regardless of which item was yellow, the bulb or the cover. You do want to make sure the signal is as bright as the previous one as it looks like that is a requirement in your state.
Sorry but legal questions demand complicated long responses......
Oh and if you have a hard time sleeping here's the link:
http://www.pacode.com/secure/data/067/chapter153/chap153toc.html