I am thinking of shooting some narrowband. I have a Ha and OIII filters. If i use the Ha for red and OIII for blue would it be possible to use the Hb for green or would this be a bad choice as the OIII and Hb are very close. Normally people use SII for the other colour.
Paul,
If you're imaging using an OIII filter (emission lines 495nm and 500nm), you've got the blue/green covered. You're right, Hbeta isn't far off at 486nm. I'd get yourself a SII filter as the next step. You can map the emission lines to any psuedo RGB channel. SHO (Hubble) or HOS (Canada-France-Hawaii Telescope - CFHT) palettes are probably the most common. You don't need to use all three filters. Two filters work well. Depends on what you are trying to portray.
