Code shared from the Rust Playground

playground.rs

// Convert number to hex string.
pub fn to_hex (value: i32) -> String {
    let value = if value > 255 { 255 } else if value < 0 { 0 } else { value };
    format!("{:02X}", value)
}

// Join all hex strings together from rgb input.
pub fn rgb (r: i32, g: i32, b: i32) -> String {
    format!("{}{}{}", to_hex(r), to_hex(g), to_hex(b))
}

#[cfg(test)]
mod tests {
    use super::*;
    #[test]
    fn it_should_be_black() {
        assert_eq!(rgb(255, 255, 255), "FFFFFF".to_string());
    }
    #[test]
    fn it_should_be_black_too() {
        assert_eq!(rgb(255, 255, 300), "FFFFFF".to_string());
    }
    #[test]
    fn it_should_be_white() {
        assert_eq!(rgb(0, 0, 0), "000000".to_string());
    }
    #[test]
    fn it_should_be_other() {
        assert_eq!(rgb(148, 0, 211), "9400D3".to_string());
    }
}