A red line doesn't mean you "must render". It's just PP's best guess about whether or not it will play back in real time without rendering.
PP doesn't always guess correctly. And even when it does, you don't always need real time playback.
Forget the colors, and only render when playback stutters and you absolutely need it not to.
Thanks for replying... The reason I know I must render is not only is there a red line, but immediately on dragging any effect (such as 'auto levels') to the time line, the playback monitor becomes black (though the audio is still fine). But any effect I apply I must then render the work area in order to see playback. Does this mean I have some setting wrong? I hope so, because this is unworkable, since even to consider any effect I must sit through yet another rendering.
Even though when I scrub at the point when the playback is black i can see all the video and once I render I can see the playback. Doesn't that suggest a setting in premiere, not an issue with any driver?
What is displayed, when you Render a Clip/Timeline is really a proxy - the Preview files.
With a tricky CODEC, and an Effect, it just might be that PrPro is not able to get that display through the video driver.
What make/model video card do you have?
What is the installed driver version and date?
Is that the latest from nVidia, or AMD/ATI?
[Edit] Do not know why I could not Reply to the OP's question directly?