Another thing you can do is soak the fabric in diluted coffee or tea. The brown water will tone down the yellow and make them look antique. Rinse once and let dry before hanging. (Vinegar in the rinse water will usually set the 'dye')
Sometimes, adding color is the key.
To calm down yellow, try light grey dye. You have to experiment a little, so get the largest scraps you can from the valances and try it out. If not grey, try a cream color.
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