Many, if not all of Nirvana's records on Sub Pop were pressed by Erika. Erika did not add anything to the matrix codes so the plant cannot be identified that way. However, "Mfg. by Erika" can be found printed on several sleeves and labels; the plant has been mentioned numerous times in Sub Pop lore; and on the testimonials page on their website there is a comment from "J" of Sub Pop, Jonathan Poneman, stating that they have worked with the plant since the dawn of grunge .
As mentioned, some pressings have Erika credits and their phone number on the labels and/ or on the sleeves. In Nirvana's case, this applies to the 1992 pressings of Bleach and Sliver (SP73). At the time Sub Pop owed Erika money, so they let them press and distribute some records . However, that Erika received all proceeds from that is doubtful. Sub Pop would still have to pay publishing fees to the band.
Back in the 1990's, Sub Pop usually did not specify the particular color(s) when ordering colored vinyl. Instead they let the pressing plant decide which colors to use, to avoid delays if an ordered color was out of stock.  Sometimes plants could use leftover color that was lying around to make odd copies of singles and albums on different colors.  When pressing the 1992 reissue of Bleach (SP34), Erika pressed around 200 white records with a light green tint (image 1). Sub Pop were concerned that these could be passed off as first pressings. Most ended up in the hands of employees, other bands, and journalists, while a few may have been sold. 
It is also possible that some records were pressed by other plants. Old rumors suggests that there were so many different colors because Sub Pop used whichever pressing plant where they could obtain credit . This may be true to a certain degree, but when pressing different colors of a certain release it seems to make sense to press them all at one plant, for quantity rebate and better use of pressing plates.
1. White 1992 pressing of Bleach with green tint