Remembering back 25 years or so when I worked as an architectural draftsman, the yellow paper printed to a purple-blue-ish line on white paper - it was quite pale. There were also gloss versions that left a black line but I can't remember if they were also 'yellow' before being exposed. We also used a stock that came out sepia brown.
Generally a print was made from a 'positive' drawing on tracing paper, the image would appear where ink obstructed the UV light from reaching the transfer sheet/copy. Anywhere with no light got exposed and would come out white-ish. Prints would fade with exposure to daylight.
The paper would be stored in black plastic sleeves in light-fast draws before use to ensure it didn't go bad. We used to go through a ton of the stuff. In the practice where I worked, we usually had one person running the print machine full time. The yellow paper version exposed to UV light was way better than the older ammonia machines, which used to leave me feeling light-headed after about 20 minutes of use.
CAD wasn't really common at that point, so we hand drafted everything, if you had to make a significant change to the drawing, it often meant starting again, minor changes were made be scratching the ink off the sheet with a razor blade and making your amendments to the 'original' sheet.