If you say the color is a little darker, that means something is happening. Some of these tests are really hard to read. When i was going thru my cycle, i made a spread sheet and tested every day and wrote the results down.
Even if you cant tell much change but notice the color is a little diffrent, write it down and after a few weeks look at the sheet and see how things have changed from day 1 to present.
Hope that helps. I agree, after 12 days you should have already seen significant change.