188.8.131.52.1.5: A major power may not conduct any trade (except through eligible controlled minor free state ports) if its national capital city (both Moscow and St. Petersburg for Russia) is occupied by unbesieged enemy factors. A major power at war with Great Britain may not conduct any domestic trade.
184.108.40.206.2.1 British Colonial Trade: Great Britain receives an additional 30 (only 15 money points while at war with the USA - see 220.127.116.11.18.104.22.168) money points for trade with its colonies.
Rule says "any trade" and colonial trade seems like a trade. Argueing about this rules seems ilogical. Spok dixit.