Yes, there can be variations in how much brown is on an egg.
That's why hens who lay brown will sometimes lay lighter or darker eggs, or occasionally even white ones.
Between changes in the amount of brown, and changes in the amount of bloom, you can get quite a variety of shades from one hen.
It does seem that some hens lay more variable eggs and some lay more consistent eggs. Of course consistent eggs are one of the things commercial layers are selected for, but olive eggers are new enough that getting olive eggs at all is the main focus in their breeding at present.