What does the yellow mean in this image? It's from https://www.viprbrc.org/brc/
Sulfur atoms are shown in yellow.
The molecular viewer that you use is JSMol (JMol ported to the web).
Atoms are colored by element: grey C, blue N, red O and yellow S.
If you wonder how other atoms would be colored, see JMol's Default element colors, by periodic table.