For 'Networks,' substitute 'operating companies.'
London Overground, MerseyRail (plus Tyne & Wear, slightly different), Transport for Wales and ScotRail are already locally or regionally devolved so will most likely retain their controlling entities' house colours.
Hull Trains, Lumo and Grand Central are open access operators and not nationalised.
Plus, in the fullness of time, other locally devolved networks will no doubt join the City Region model like MerseyRail - Andrew Burnham's 'Bee Network' operation for Greater Manchester being the most vocal, West Midlands being next, in having their own local identities.