Grace has a lot of fun explaining colors to Eridians. Telling everyone what rock or gemstone their carapace reminds them of is easy, low-effort small talk which, if desired, can shift readily enough into a scientific or cultural discussion. A nifty little alien trick that has nothing to do with vulgar secretions or consumption. Not to mention it helps his still malnourished brain remember all the new Eridians he's been meeting.
That is until he informs an Obsidion-esque Eridian that they are black. Obsidian asks the common follow-up question on whether or not that's a 'good' color and Grace (white) has a MASSIVE crash-out about the fact that he went to an alien planet and immediately started introducing new forms of racism. It takes a whole team of ethnologists (also the invention of vitamin B12) to convince him that it is literally not possible for Eridians to integrate 'color' into their culture in any meaningful way, and also that while what he's 'seeing' (the absolute outermost layer of carapace mineral composition) is partially genetic, it's also fairly heavily influenced by your diet and local soil composition from between Iℓℓ to λλℓ- years ago (depending on age and professional/regional erosion rates) and is therefore both culturally nonsensical as well as likely to fluctuate over time. Also that Eridians have their own cultural baggage of things that they judge each other for that Grace can barely perceive. All of which gives Grace's new things to be worried about, but at least makes him feel better about not doing accidental colonialism.