If you are editing HD video and you want accurate color representation, you will need a breakout box like this:
https://www.matrox.com/video/en/products/mxo/
Simply using a second computer monitor attached directly to the computer (i.e. as a second computer monitor) will not give you accurate color preview for video content. You need a proper video output interface and a way to properly calibrate the preview monitor.
If you are not editing HD, you can use Firewire out of your iMac through your camera or DV deck to connect to a TV. Alternately, you can use a DV interface like the Canopus ADVC-110:
https://www.grassvalley.com/products/advc110
The Canopus is actually preferable to a deck or camera because most consumer DV devices (cameras and decks) do not output the proper black level for US-NTSC, which is 7.5 IRE. Instead, they mostly output 0 IRE, which is too dark. If you do color correction while viewing improper black levels on your preview monitor, this will result in your videos appearing washed out when you burn a DVD or play back your video on a properly calibrated system.
More info here:
https://www.signvideo.com/dv-black-levels-dvd-authoring-mpeg-2-part-1.htm