This is doing my head in if they're so focused on having specific colours for the Xlines. (instead of doing the cost saving thing of all black & gold from the get go).

This is what I would have done from the off;
X1 - Pink (to complement Connections 4)
X5/X15 - Blue (to complement Durham Diamond 16/16A - now abandoned)
X10 - Black
X6/X20 - Purple (to compliment Prince Bishops 20)
X21 - Light Green (to compliment Angel 21)
X30/X31 - Dark Green
X45/X46/X47 - Dark Green
X70/X71 - Dark Green
X84/X85 - Yellow

This could then be (with the changes to routes X5/X15/X6/X20/X46/X47/X84/X85);
X1 - Pink
X10 - Black
X20 - Purple
X21 - Light Green
X30/X31/X45/X71/X72 - Dark Green

But i would've been easier to paint everything into gold and black or red as the black is the smartest and red fits with GNE's livery.