"I figured it was best to start low, and build up as the experiments go on," Theo teased, then nodded as Senku went on. "It's the same in Potions. If you don't add the precise amount of something--even something simple like salt--you're likely to melt your cauldron. And that's the best-case scenario." This part of Senku's science was familiar at least.
He glanced at Senku's finger with amusement, then thought it over. He wasn't familiar with the chemical names, but Potions had been one of his best subjects and he could make educated guesses. "Well, given what I've observed when working with salt as an ingredient for potions, I'm going with yellow, or somewhere along the yellow-orange spectrum." He paused a moment, then continued, because he knew Senku would want some sort of explanation for the guess. "There was a student in my potions class who once missed the cauldron when adding the salt, and the resulting disaster was distinctly yellow-tinged."