We'll extend our random-number-generating service with two features – generating random colors and shuffling an array of bytes. For the first feature, we need to add the Color struct to hold the color components. Create a new file, color.rs, and add the following code to it:

#[derive(Clone, PartialEq, Eq)]
pub struct Color {
    pub red: u8,
    pub green: u8,
    pub blue: u8,
}

Add two constant colors that we'll use later:

pub const WHITE: Color = Color { red: 0xFF, green: 0xFF, blue: 0xFF };
pub const BLACK: Color = Color { red: 0x00, green: 0x00, blue: 0x00 };

The struct also implements PartialEq and Eq to compare a value with these constants.

We'll use a textual representation of color that's compatible with CSS. We'll support RGB colors in hex format and two textual colors: black and white. To convert a color to a string, implement the Display trait for Color:

impl fmt::Display for Color {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self {
            &WHITE => f.write_str("white"),
            &BLACK => f.write_str("black"),
            color => {
                write!(f, "#{:02X}{:02X}{:02X}", color.red, color.green, color.blue)
            },
        }
    }
}

This implementation writes three color components to a string with the '#' prefix. Every color component byte is written in hex format with the '0' prefix for nonsignificant digits and with a width of two characters.

We can now use this formatter to implement the Serialize trait for the Color struct:

impl Serialize for Color {
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: Serializer,
    {
        serializer.serialize_str(&self.to_string())
    }
}

This Serialize implementation calls the serialize_str method of the Serializer to store a hex representation of a color to a string. Before implementing a custom deserialization, add all necessary imports to the color.rs file:

use std::fmt;
use std::str::FromStr;
use std::num::ParseIntError;
use serde::{de::{self, Visitor}, Deserialize, Deserializer, Serialize, Serializer};