Improve color of banana jam?
A splash of lemon juice mixed in the banana right off the bat can fix that colour at the beginning, but the molasses will turn it brown afterwards... 

So if you want it lighter in colour, you'll need to substitute the molasses... Can you use white sugar and throw in a bit of rum in the end ?  

If you must keep the molasses then just go for the darker route... Perhaps let the banana oxidize as much as possible before mixing ?