The Electrolux Model B-8 was manufactured from 1963-1983--first as just a polisher/scrubber. The Carpet Beautifier component was added in 1965 as an option which later became standard. Your shampoo dispenser tank is from 1965 or 1966--the only one to bear the sparkle graphics (until they figured out that carpets don't sparkle when they're clean and removed them--ha). I would guess, therefore, that the base of the machine is aquamarine green--as Electrolux called it.
Electrolux Corporation sold its registered trademark back to AB Electrolux of Sweden and has since been called Aerus LLC. Your best bet in finding a replacement part for your parents' shampoo dispenser tank is to contact some Aerus stores near you that may have still have some old stock. You will find a store locator at aerushome.com. Also, in case you're interested, Aerus still sells the Turbo Shampoo and scrubbing & polishing pads (which are also used with the updated version of the B-8 of its current product line).
I contacted the main office a year or two ago and was told that I could purchase a new tank but not individual parts for it. I don't recall the price, but it was rather high. However, if it lasts 50 years or so maybe it's not such a bad deal! Contact the main office if you're in need of an owner's manual; it will provide a scanned electronic copy of one for you free of charge.
You could also check eBay, because I've seen just the cap and tube assembly listed periodically.
Best wishes in your endeavor!
