Personally, I think Hornby have absolutely nailed it with the shade of green they have done this time. It has depth and life unlike the shades used back when the Britannia came out circa 2006. I understand that colour perception is a highly subjective thing, given the vagaries of the human eye and brain, but compared to photos of the prototype and seeing full-size BR green locos in preservation, with correct application of BR green, to my mind if not spot in it is at least very, very close
OK, those lined green cylinders and the firebox are a bit of a fantasy but nothing some well aimed black paint and weathering won't solve if you have the skills.