Many of the characters in Undertale use colored text to convey extra semantic meaning - oftentimes red text denotes something dark or negative, for instance. One thing that entertains me is how Sans uses colored text to give the human subtle hints about things he knows but is not saying outright. His words on their own could be misleading, but extra nuance and knowledge can be gleaned by looking at the colors.
At Grillby’s, Sans asks the human if they know about a talking flower. He asks this question in yellow text.
However, once he starts talking about Echo Flowers, he uses blue text to mention those flowers. He’s making a distinction between the Echo Flower and the talking flower he first mentioned. The ending of Sans’ speech about flowers is about a flower that is acting strangely around Papyrus, and he tells Frisk to be careful. This suggests that Sans brought up the conversation not to talk about Echo Flowers, but to subtly tell you he knows about Flowey and warns you to be careful around him.
Sans also does a subtle hint that there’s a difference between regular blue attacks and Papyrus’ “very special attack.”
The blue text when Sans says “special attack” is a different shade than when Sans talks about “blue attacks.” He’s doing the same sort of color distinction as with the yellow and blue texts in Grillby’s about the flowers. If you’re not paying attention, you think he’s talking about one and the same thing. But if you pay attention to the colors, you realize he’s actually changing topics.
The light blue hue Sans uses to talk about blue attacks is the same blue hue seen in various characters’ blue attacks. And the darker blue that Sans uses to talk about Papyrus’ attack is the color the SOUL turns at the start of Papyrus’ Encounter.
So, even though Sans’ words seem misleading and can make you falsely think Papyrus just uses blue attacks, Sans is subtly also hinting that Papyrus is going to do something more. He’s going to turn your SOUL blue, which will make your SOUL feel “gravity” toward the bottom of the Encounter box.
And right before you go into Papyrus’ Encounter? The color he is using to talk about blue attacks is the dark blue.
Sans foreshadows with text color. He implies information with text color. He trolls with text color, even. Rather than saying things outright, he gives information to you subtly. In the case of Flowey, he’s hoping you can pick up the subtle hints he’s giving, so that even though the conversation sounds normal in public, you can carry away caution and knowledge for the rest of your journey.