Ok, so i will admit there is some difference between Imperialism and Colonialism. FDR couldn't aggressively pursue a policy of colonial expansion at the time. He couldn't join the war (on either side) and was doing everything possible to support the allies without officially joining the war.
But when it comes to Japan and China specifically, the US wanted access to Chinese markets, (something the Chinese had fought wars to prevent, notably the Opium wars in the 19th century) and having made gains during the boxer rebellion (for the international force including the US, Europeans and Japanese) Japan was now threatening those gains - and US interests in south east asia.
FDR couldn't declare war on Japan, but would have if he could, instead provoked the Japanese by ecomonic embargo.
The attitude towards China is one of Imperialism, a lack of respect for Chinese socereignty (the middle kingdom had traditionally been self-sufficient and didn't need to trade with the rest of the world, and the Europeans, US and Japanese didn't respect this - they wanted to take advantage of the situation)
At the end of the war, Japan was disarmed, occupied by American troops, and completely open to US trade. They effectively became a US colony by any other name (you couldn't really call them an ally after you disarm them...)
FDR was anti-colonialism so far as it was in US interests to oppose Japanese and European Imperial ambitions.