what's the china fir, is it a kind of fir wood?.

no, it is not a kind of fir wood though it called china fir wood, its genus name is Cunninghamia honours, for its details, pls see the blow description in Wiki.

/wiki/Cunninghamia

is it can be customed?

yes, we can custom it as your requirement

how long it can keep well in outdoor environment?

normally more than 5 yeasr even without any paint, china fir wood has great ablity to keep well in outdoor

could I stain color on it? or can you make color for me?

no problem, we can do it, you as well.