I just imported a fully mastered Hip-Hop song released in 2003 (Joe Budden - Pump It Up, if that's relevant), and I noticed that the RMS meter lit red (but not the regular dBFS level meter). The mix sounds fine, though. What is the target RMS level one should go for when mastering? I thought anything hitting red was off-limits.
RMS is old school now. You should preferably use a plugin that measures the loudness in LUFS or LKFS. These are the same values only in Europe they use the name LUFS and in USA LKFS. For a target loudness level for Hip-Hop I think they produce very loud like -8 LUFS (that's about -6 RMS) but in general a -10 LUFS value should work okay. However for streaming, mixes sound better if for Youtube are mixed around -13 LUFS or -14LUFS for Spotify because levels louder than these will be punished to a lower level around Youtube or Spotify loudness target. You RMS meter goes to red perhaps because it's calibrated for a lower value or you have imported an mp3 and has some distortion from the conversion.