Red light will pass through a red lens with no reduction. White light will
have all but the red component filtered out.
The red LEDs will be a deeper red, as against a white bulb.
However you do have to be careful that the bulbs are bright enough, some are
not.
Also it is likely that the indicators will not work as the system will think
that a bulb has blown and flash at twice the normal speed. You can either
fit load resistors in parallel with the bulbs, or (if your vehicle permits)
fit a special flasher relay.
http://www.ultraleds.com/ have a good range of LED replacements, and they do
give an indication of how bright they are.