Vintage linens, even well cared for or newly washed linens, sometimes have yellow spots or stains that are often caused by a poor rinse.  To get rid of the majority of these stains mix:

1 Gallon warm water (warm enough to melt the soap)

2 TBS of GRATED Ivory Soap  (buy it by the bar and use an old food grater to shave off 2 Tablespoons)

1 TBS Bleach Mix 

Stir and melt the soap then let cool until liquid has gelled a bit. Apply the gel to the stain. Make sure to test a small area first before using.  As with all vintage pieces, rinse very well, over and over, to remove every trace of the soap.