The painted flowers in this guide are dyed with cosmos sulphurus, but you can you any other plant material you like. Other yellow colours for your natural dye adventures are:

St John’s wort. I like to forage these wild flowers for brilliant yellow colours.
Scotch broom. In spring the mountains are coloured yellow from the blooming scotch broom. Pick the flowers and your fabric will turn bright yellow.
Weld. This plant will give you a lemon yellow colour.
Onion skins. Different types of onions give different shades of yellow. Red onion skins give a more golden colour.
Marigolds. Right outside my studio I have planted marigolds to pick whenever I like to dye golden and orange colours.