Hi Jeg and welcome!
I am also a convert from SW, it worked for me for a few months but not after that.
Your dietician's advice is sound and those are good proportions to follow so that you don't have to weigh and measure everything. I don't have the Collins Gem book (it sounds like it doesn't have much introductory information!) but usually the traffic light systems are:
green - eat as much as you need to
yellow/orange - eat occasionally/in moderation
red - avoid except for very rare occasions.
There's an explanatory chart here:
The GI Diet - Glycemic Index Traffic Light Food Chart
and a useful overview (with a link to a sample day's menu at the bottom) here:
GI Diet Report
The focus is on eating more 'good' low GI carbs (green) and avoiding higher GI carbs (yellow/red).
I would say that regarding your question of whether you can eat as much as you like of green foods, I have found GI to be different to SW in this regard. On SW you are encouraged to eat a lot of food whereas on GI you are encouraged to eat 'enough' food in 3 meals and 2-3 snacks in order to keep your blood sugar stabilised (which is one of themain points of low GI). Carb portions (rice, pasta, sweet potatoes etc) should be limited to recommended low GI serving sizes.
If you're after a really simple intro to low GI eating, I rate Rick Gallop's GI Express Diet book highly. He also uses a traffic light system (so you can get use out of your Collins Gem book) but also gives suggested portion sizes for carb foods and simple meal ideas and quick recipes. You might be able to find this in your local library if you don't want to buy any more books. Also, everyone here is really helpful so just ask any questions you might have or peek in our diaries to see what we're all eating.
