I am not much on folklore or handed down recipes, so forgive me,
butwhat possible purpose would vinegar serve in this equation?
You can easily use the food colors the way they are right out of the
bottle. If they are too strong, dilute them with isopropyl alcohol,
the kind sold over the counter. Look for 70% strength, but the 50%
should work find if that's the stuff in the cabinet.
This will not be very color fast, but if it is for inside use, it will
work fine. To help set the colors, use a coat of clear exterior
finish such as clear enamel from a rattle can.
We have been coloring our stuff in the woodturning club this way for
years. Talk about tight... those guys wrote the book. I showed them
how to color their turned ornaments with red and green dye and it has
been a hit ever since then.