Just opened 2 packs of WLP500 with a Best By date of 4/15/20 (mfg date 10/17/19, 86% viability per my calcs), and both liquid packs were med-dark brown. I'm used to seeing a creamy white or off/white yeast slurry, and I'm concerned that the yeast might be dead, or partially dead.
Just pitched a 2L starter with Light Pilsner DME, and the starter wort is brown. There was no funky smell when packs were opened and pitched, but no yeasty smell either.
I just got off the phone with WL, and they say this is normal, as they sometimes switch out their propagation wort from DME to grain-based and vice-versa. Seems a little weird to me, as I assumed labs separate yeast from propagation starter before release, just as many homebrewers do with water after harvesting.
I've heard from a local pro brewer that large yeast mfgrs sometimes dump bad batches on the Homebrew market - though WL seems to be pretty upstanding. They have dropped the mfg date on their packaging (very common these days in food-and-wine packaging) - really bugs me with packaged craft beers (don't get me started on coffee roast dates!) - but they are very transparent about the mfg (QC) date if you take the trouble to enter the printed Lot# on their yeastman.com website (QC link)
I'm going to wait to see if the starter takes off, but I have no reliable means to get a cell count for pitching if the starting viable cell count was off. WL said they stand by their product and offer a replacement guarantee if it is defective, but I hate to ruin a whole batch finding out.