Yes, violet is rare. Violet (or purple) is an added colour meaning that not a colour mutation by its own right. It must be seen visually to have violet colour, otherwise, none exists.
Violet can be single factor violet or double factor violet. Singles mean light purple colour while double tends to intensity the purple colouring more. It is hard to see violet in green budgies but in blue budgies, it shows more prominently. Violet shows in the body and not only in the neck area.
I don't have any violet in my flock. But I saw some brilliant violets in the petshops here and not buying at the moment.
All of my birds have a violet factor. Some have the grey factor with the violet factor so it is harder to see the violet.
It is best seen in my skyblue hen and my cobalt male
I thought it wasn't rare since I had it in all my flock, but I know it is hard to see it visually sometimes.
I don't think violet is rare, at least from the numbers of violets I've seen posted in forums and the fact that they have shown up in my local pet store.
I think it's partially because violet is semi-dominant, meaning it's quite easy to breed a violet budgie and get violets in your nest. As has been mentioned, violet shows best in the blue series birds, with cobalt being the best choice.
I have not seen violet budgies for months selling in the petshops. If exist violet budgies during my acquiring time, I would have grab one or two violets. Right now I have no violets in my flock of 8. This is the reason I said rare in the 1st instance.
I see violet in my skyblue greywing opaline yellow face male, but it is hard to see in pictures. He has it in his longer tail feathers, the back of his head, and what is left of his flight feathers. He is so pale that it is hard to see, but I am posting a photo anyway. Opinions welcome. If needed, I will take another photo when he wakes up tomorrow.