Certain colors develop an "onion" smell when thinned with tap water. I have painted with acrylic paints, yours and other brands, for 30 years and have noted this phenomenon for a while, but since the smell went away when the paint dried it was never a problem until recently. Now, for some reason, the painted surface (various rice papers) still retain a strong onion smell, even when dry. I am finding that phthalo green, phthalo turquoise and some blues are the worst offenders. I may try using distilled water to thin these colors and hope that solves the problem. Do you have any ideas?
We believe the onion smell is caused by several factors. First, when you dilute acrylic paints with either tap or distilled water, you eventually are going to encourage mold growth. While GOLDEN does add biocides to kill off mold growth, when paint is greatly reduced and stored, the biocide level is reduced to where it is ineffective. Also, pH is important in controlling bacterial growth. We maintain an alkaline pH (around 8.8 for most products). Water is generally around 7.0 and in a wash, the pH is normally in the high 7's. When you lower the pH of the paint, you encourage microbial growth.

The other part of this is the kind of pigment, which you mentioned with the Phthalo colors. Phthalos have a distinct smell to them, and it may be that this distinct smell, coupled with some bacterial growth is creating the onion smell.

You can get Liquid Preservative from us to add to the watered down paint mixtures, and you can also add some plain household ammonia to the mixtures to raise the pH. Generally a capful to a quart is sufficient.