Giving you the M2TW version, don't play RTW any more.
Go to descr_sm_factions.txt in your data folder. Back it up(Always do that)
the entries will be of the following:
england, faction england, spawned_on_event
culture northern_european
religion catholic
symbol models_strat/symbol_england.CAS
rebel_symbol models_strat/symbol_rebels.CAS
primary_colour red 215, green 0, blue 0
secondary_colour red 0, green 87, blue 177
loading_logo loading_screen/symbols/symbol128_england.tga
standard_index 12
logo_index FACTION_LOGO_ENGLAND
small_logo_index SMALL_FACTION_LOGO_ENGLAND
triumph_value 5
custom_battle_availability yes
can_sap no
prefers_naval_invasions yes
can_have_princess yes
has_family_tree yes

The bold areas need to be same, i.e

primary_colour red 215, green 0, blue 0
secondary_colour red 215, green 0, blue 0