I used Tneon b4 to veg under, it was 9500k
And it was just fine! Have to mention that this was TCL 110w
I also used the 6400k on the same fixture , was also fine!
Back to MH, I used philips 400w which I think was 4500k
Also used Lumatek MH 400w and 600w
While both suppose to be 6400k.. My eyes capture colour difference between both, the 600w to be alittle more yellow..
All was just fine!!
My eyes likes the white colour and for sure I feel it gives shorter internode spacing.
But what the plants like?! No Idea and I have always wonderd the same as well..
So I would say any thing from 4500k up to 9500k would work fine!