I have kept the shawl I knitted for my daughter, many years ago and when I looked at it recently I saw that it was rather yellow. Washing has improved it but its still a little discoloured, does anyone have suggestions to improve this? It is over 37 years old so I want to treat it gently. It is knitted on two needles in Patons 2ply pure wool, the pattern was from a Patons leaflet. The shawl is joined when knitting is completed.