POD in the 1840s- uber-Manifest destiny, Mexican War results in annexation of Mexico, William Walker Nicaragua, plantation Cuba, etc. Only British Honduras escapes the tide of annexations which stops at Colombia.
America has its hands full with all these colonies, so Seward doesn't buy Alaska. Instead, it goes independent in the RCW, and is settled by exiled White Russians.
I won't try and explain why the US is red, as I don't know if you mean anything by it.
As for the colonies- again, date of map or butterflies.