3 Cent Perf 12 Washington

I have these 3 perf 12 stamps that need to be checked for watermarks, from what I see online many say the new watermark fluid doesn't work well and many say they use lighter fluid. what would you recommend I use?
The last one of the three stamps has a blue color and I was wondering if it could be the gray/blue paper rarities.
Thank You for your time.
James

As you surmised the watermark will be the best method for determining which stamp is which.Looking at the scans of the backs, they all have the same color paper. Originally white, now toned to a yellowish. I use Ronsonol lighter fluid for watermarking and it will work just fine for these. The ones that are difficult are the orange colored issues, and they're just as difficult with watermark fluid as with lighter fluid.