Given that 4 of 6 are white, then there would seem/could to be a fuel issue-could be injector issues as mentioned. Change the plugs at minimum(no matter what) and have a professional take a look at the problem(Hopefully there is a Mercury certified mechanic in your area that you are comfortable with). Be aware though, that to truly consider the lean/rich condition of a motor(based on looking at the plugs) that you need to run the motor at WOT and then shutdown to get a "true" reading. Not the safest and not a method that I would recommend. We all may have many "good" suggestions but we are behind the keyboard and can not solve the problem directly. IM your email and I will send you .pdf copies of the Mercury manual for your motor if interested(Good reading and educational-LOL). I have the same motor as yourself and a continuing lean condition is a recipe for disaster and $$$$$$$$$. JMHO though. Good luck in all cases!