Its thallus is thin, making it less sturdy. The blade has a more lanceolot shape on calmer shores. Iridaea also varies its pigments according to light availability, and this makes its color a variable characteristic. The thallus appears darker red under low light levels because of increased levels of phycoerythrin, which assists photosynthesis. The thallus can range from dark purple to almost black when growing very low in the intertidal and subtidal where light is scarce.
Individual plants are often more red towards the stipe where the blade is most shaded.
When growing on the sides of rocks instead of their top surfaces, the thallus also darkens because it spends more hours shaded. Photobleaching contributes to color differences because plants growing in high light levels get bleached by excess light as well as producing less phycoerythrin. Color varies a great deal even among specimens growing on the same rock surface. For instance, when Iridaea grows shaded by Pelvetia , it assumes the dark red tone of a lower tide-depth specimen.