The color 'orange' is some combination of the primary colors 'red' and 'yellow' and can range in appearance from red to yellow depending on the amounts of each dye. If Brenda or some other woman was wearing such a sweater, was in the VD home and then was in contact with DW or contacted someone who then contacted DW, this could explain the orange acrylic fibers. Acrylic is very well known for 'static' which would make the transfer of numerous fibers more likely than not, especially in a dry climate like San Diego.
The prosecution did analysis and showed the jury those items could not be the source of the orange fibers.
One of the shirts in question was, in fact, red, so you have good eyes!