BTF
You only (usually) need to pass an approved colour vision test at the initial medical and will never have to take another.
It is seen that CVD is an inherited trait that does not change with age. In fact colour vision for everyone can change with age, but that bit is ignored and as it is usually a yellow deficiency, would not show at ishihara or probably the lanterns. Interestingley, it would in the new CAD test!!
If the examiner feels there are clinical grounds that you might have diminished colour vision say because of disease or poisining etc then they will retest you, but I think it is very rare.
If you have passed the colour vision of your initial class 1 then you will not have to take any more tests when the CAD is introduced. If you failed all the CV tests, you should be allowed to try the CAD when it comes in which if you pass will remove colour vision restrictions.
I did touch upon yellow dficiencies. These are not screened by ishihara or lanterns so someone with a yellow or blue problem can pass - at the moment! The new CAD test will show these as well as red/green. In fact the CAA have told me that they are looking to replace ishihara with a shortened version on the CAD. The CAA do however not see yellow or blue as critical in aviation! humm taxiway lights??