You are at a top LAC. You are considering transferring to another top LAC or a top private university.
If Barnard and Northwestern would be affordable – the first test – I would still only transfer if there is a compelling academic or socioenvironmental reason.
If you can’t find a really compelling reason, I would stay at Colby. It’s a great school.
You have three wonderful options. I consider the schools to be academic peers.
Assuming they are all affordable choose the one that feels like the best fit for YOU. This is a case where three reasonable people could make three different choices.
Do keep in mind that transferring successfully will require a real effort on your part – many friendships are formed freshman year when everyone is new to the school and you will need to develop new routines while others are already in a groove. Transferring can be worth it (FWIW I transferred after a year) but should be considered only if you feel the new school will be materially better for you in the long term.