The Ishihara (my spelling) colour test is a poor indicator of colour difficiency. If it is important to you to resolve that issue it is possible to have a 'lantern' test which is more accurate. Telephone the CAA medical department who will help and advise on that.
I wouldn't bother unless you want to do instrument flying or night flying. If you are anything other than slightly colour blind then you will likely fail the lantern test and will have paid out a fair sum of money. You can fly completely safely even though you are colour blind and many people do. Colour is not used that much today (no coloured lights at airfields as most people use radio). I am colour blind and the main area I find difficult is being able to see the different colours on a chart such as Blue vs purple etc. So, if you can manage the chart you'll be OK.
I think the NPPL is perfectly safe as long as you have the right attitude and are careful. This applies equally to a std PPL. As others have said, it isn't the licence thats safe, it is the pilot, and that is down to you and your instructor.
Get your doctor to give you an opinion as to whether he thinks there are any medical issues. The NPPL website has some good guidance notes for both you and your GP. I suggest reading the NPPL website and the links on it carefully so that you can form your own opinion before paying a doctor to tell you what you may already know. Then, if there are any areas you are unsure about get some advice.
I do think the NPPL is very well thought through. The different medical requirements were created by experts at the CAA and elsewhere who looked at the facts and figures before establishing the criteria. Their view is that it is just as safe for normal day VFR single engined aircraft.
ZA