This may have already been explained in the past but what significance of the purple Class 66’s on SEHS?
There are no purple Class 66s on SEHS. The only Class 66s to have been in purple were the FirstGBRF ones years ago, but even that's technically blue...
If you have created a custom livery (or attempted one) then they will appear. I created a test class 66 and it appears on SEHS at Hoo Junction every time I play. It should go away if you delete any custom liveries and will default back to the EWS one.