Weld

Weld is a grassy plant yielding bright clear yellows.  It is considered by dyers to be among the fastest yellows, meaning that it holds up well to both light exposure and washing.  Weld and woad was allegedly used to create the Lincoln Green color that Robin Hood's men wore during their time in Sherwood Forest, so if you want to be authentic....

Weld under indigo will create a gorgeous bright Kelly green.  

Weld gives it's best color with a slightly alkaline dyebath, so plan to add a bit of washing soda or calcium carbonate when you dye.