EMB145 First Officer reports possibly crossing the hold line for Runway 24R after landing on Runway 24L and exiting at Taxiway D4. Poor sign illumination is cited as a contributing factor.

Narrative

I was the First Officer and pilot monitoring for the flight. Arriving at CYYZ we landed on Runway 24L and Tower told us to exit on [Taxiway] D4 and hold short of [Runway] 24R. Taxiway D4 is a high speed exit on the right side of the runway and while we were on the taxiway we noted an aircraft taking off and immediately stopped. At the same time Tower told us to; 'Stop; stop; stop.' We were still on Taxiway D4 and not on the actual runway surface of 24R. It was immediately evident that they suspected we crossed the hold short line for 24R and quickly had us cross the runway after the traffic we noted had departed. As I briefed before; at no time did our aircraft go onto the actual runway of 24R and the departing traffic was insight during this whole occurrence. Recalling the event; because it was dark and the lighting was dim with a combination of poor signage; it was not clear that we had exceeded any hold short line. If by chance we had exceed the line; at no point was our aircraft on the surface of 24R when both my Captain and I had the departing traffic insight. I believe that the cause of the problem was poor signage and lighting at CYYZ. I believe the signs there are not lit from within so that the whole sign lights up like in the U.S. Instead the signs have colored bulbs displaying only the numbers/letters so it is different and not as visible as the whole sign with colors; etc. being lit up. I would recommend [that] the in-surface lighting on the high speed taxiway be enhanced and also the signage [should] be better lit than it is to increase safety for pilots transitioning from a high speed taxiway to the critical hold short area.