It's the nic that generally creates the dark colour. Saying that, I've some custard at 50/50 with 9mg that is still clear after 6 months
I take it you just recently added nic to it? As it would've turned darker. My mix is also a 50/50, but I only make 70/30 now. So that bottle was just sitting there for ages without nic.
Normally my custard mixes get darker after a good 3-4 weeks and tastes great