FAQ: Why does my non-colored candle turn yellowish?
Soy wax in its initial state comes from the manufacturer in an
"almost white" flake form:

If we were to melt these flakes and do nothing else, they would
cool and solidify into a white mass of the same color. However, in
making candles we add things to this melted wax like fragrance oils,
UV inhibitor, etc, and these additives have an effect on the color
of the wax after it cools. Even without adding dyes or pigments to
make the deep colors you normally think of, the chemistry of the
additives creates coloration. Take a look at these two fragrance
oils:

Given the difference in coloration of these two fragrance oils
(Lavender on the left, Jungle Fruit on the right), would you not
expect one of them to have a tinting effect on the white wax flakes?
In addition to tints imparted by the fragrance oils, strong UV
rays will also turn the soy wax flakes a yellowish color over time.
Even after adding a UV inhibitor, which we do with all our candles,
if exposed to direct UV for long enough, the wax will begin to
yellow.

This candle is made with the Birthday Cake fragrance oil and it has
been several weeks since it was poured. The Birthday Cake fragrance
oil is a very heavy and dark oil, similar to the Jungle Fruit you
saw pictured above, so it added a tint to the final product. It
SMELLS wonderful and it burns beautifully, but as you can see it
can't be a perfect white. There are ways to force a candle to be
white, specifically by adding a pigment like Titanium Dioxide. The
problem with that is that the pigment particles do not dissolve in
the wax and will clog up a wick, which will adversely effect the
burn quality.