The color of resorcinol-formaldehyde resin
The polymerization of resorcinol creates multiple (spaghetti-like) chains containing the aromatic rings of resorcinol.  This is a simultaneous growth in multiple directions. Eventually, the chains trap each other in the solid state. In the analogy used, it is like stirring a pot of spaghetti and then freezing the content.
It may happen that an arene ring is just in proximity of an arene ring of an other chain.  Given a growing polymer chain may be flexible, it may happen that there are intramolecular &quot;close couples&quot;, too.  If these arenes are close enough to each other, their $\pi$-electron systems may interact with each other as if they were coupled as one.  This extension of the $\pi$-electron system may move the observable absorption band toward longer wavelengths. Then,

If there are just enough of these &quot;close encounters&quot; per unit of volume with the same arene-arene distance, the perception of color may be intensified (e.g., pale yellow -&gt; bright yellow).

With variation of the arene-arene distance, the intermolecular interaction of these $\pi$-electron systems equally varies. This may yield the absorption of light of different wavelengths / the subtraction of different colors from the white light by these arenes.  Thus it may happen that the color of the polymer is not uniform. If the range of light absorption is wide enough, the polymer may appear just black.