The best thing to do would be to get about a qt. of yeast from a brewery. I assume you’re talking about dry yeast. In that case I’d use about 3 packs.
Yeast slurry is what you would harvest from the fermenter during or after fermentation. The idea is that to restart a stuck fermentation, or to clean up a beer that was racked too early, you want a lot of yeast, and you want it to be relatively fresh. If you don’t have any slurry or anyone around who could give you some, your best bet is to make a new starter and pitch it at high krausen.
It’s probably me but what color on the fermometer is the actual temp- I get shades of brown, green, and blue spanning about 6 degrees (that’s what I meant by lacking accuracy). I’ve always gone with the middle one but that’s a guess