Is it normal that the leaves are getting this colour?

High ultra_violet, There are a few signs from overfeeding and nutrient lockout. But you are almost ready for harvest so I would not do too much about it. You will harm more with trying to fix it then just let them ripen for the last 2 weeks. Happy growing and happy harvest buddy!

Yes it's normal.

Depends on the strain and can be normal. Plant looks ok to me, so i'd say

It is absolutely normal for late stage flower to develop dark colors in the leaf. A lot of places are actually breeding that into the plant because it’s turned into a desirable look. I did a quick search for the RS11 plant and it looks like the majority of all the end of flower plants have a very dark purple leaf. I would say you are right on track and keep doing what you are doing. It should turn some lovely color by the time you are ready to harvest. Hope this helps.

Purple genes in first two and last. Fade and lack of phosphorus in third photo ... but its all in natural phaze of canna plant. Would not worry about anything !