diff --git a/src/cli.rs b/src/cli.rs
new file mode 100644
index 0000000..8b4ad89
--- /dev/null
+++ b/src/cli.rs
@@ -0,0 +1,70 @@
+use std::fmt::Debug;
+use std::num::ParseIntError;
+
+use clap::Parser;
+use image::{DynamicImage, ImageReader, Rgb};
+
+use crate::Colour;
+
+#[derive(Parser)]
+#[command(name = "imagePicker")]
+#[command(version = "1.0")]
+#[command(about = "Matches colours to their closest equivalents in a given image", long_about = None)]
+/// Matches the colours in one image to either a list of provided colours or a different image.
+///
+/// For each provided colour (either explicitly or in the second image), finds the closest colour
+/// from the first image, using redmean distance.
+pub struct Cli {
+    // The source image from which colours should be picked.
+    #[arg(value_parser = valid_image_file)]
+    pub image: DynamicImage,
+
+    // Optional second image; if used, all colours from this image will be matched to their closest
+    // equivalent in the first image.
+    #[arg(short = 'i', long = "image", value_parser = valid_image_file)]
+    pub second_image: Option<DynamicImage>,
+
+    // Optional list of hex values; if used, will match each value to their closest equivalent in
+    // the first image
+    //
+    // Should provide valid hex codes only: '24274a', and '#cad3f5' are both valid.
+    #[arg(short = 'c', long = "colours", value_parser = valid_hex_code)]
+    pub colours: Option<Vec<Colour>>,
+}
+
+impl Debug for Cli {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("Cli")
+            .field("image", &self.image)
+            .field("image_two", &self.second_image)
+            .field("colours", &self.colours)
+            .finish()
+    }
+}
+
+fn valid_hex_code(s: &str) -> Result<Colour, String> {
+    // accept leading '#' for hex values
+    let start = match s.chars().nth(0) == Some('#') {
+        true => 1,
+        false => 0,
+    };
+
+    let u8s: Vec<u8> = (start..s.len())
+        .step_by(2)
+        .map(|i| u8::from_str_radix(&s[i..i + 2], 16))
+        .collect::<Result<Vec<u8>, ParseIntError>>()
+        .map_err(|e| format!("{}", e))?;
+    let rgb: [u8; 3] = u8s
+        .try_into()
+        .map_err(|_| <&str as Into<String>>::into("Hex string must decode to exactly 3 bytes"))?;
+    Ok(Rgb(rgb))
+}
+
+fn valid_image_file(s: &str) -> Result<DynamicImage, String> {
+    ImageReader::open(s)
+        .map_err(|e| format!("{}", e))?
+        .with_guessed_format()
+        .map_err(|e| format!("{}", e))?
+        .decode()
+        .map_err(|e| format!("{}", e))
+}
diff --git a/src/main.rs b/src/main.rs
index b56a430..f672dcc 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,11 +1,15 @@
+mod cli;
+
+use clap::Parser;
 use std::{
     collections::{HashMap, HashSet},
-    env,
     num::ParseIntError,
 };
 
 use dotenv::dotenv;
-use image::{DynamicImage, GenericImageView, ImageReader, Rgb};
+use image::{DynamicImage, GenericImageView, Rgb};
+
+use crate::cli::Cli;
 
 // TODO (medium priority): replace hashset with another hashmap<u8, Vec<(u32, u32)>>;
 // that way we can track pixel locations for colours.
@@ -45,7 +49,7 @@ fn extract_colours_set(img: DynamicImage) -> HashSet<Rgb<u8>> {
     out.into_iter().map(|c| Rgb([c[0], c[1], c[2]])).collect()
 }
 
-fn closest_colour(colour: Colour, all: &ColourSet) -> Option<Colour> {
+fn closest_colour(colour: &Colour, all: &ColourSet) -> Option<Colour> {
     // use redmean to calculate the distance between colours
     // inevitably O(len(all))
     let mut closest: Option<Colour> = None;
@@ -71,31 +75,23 @@ fn closest_colour(colour: Colour, all: &ColourSet) -> Option<Colour> {
     return closest;
 }
 
-fn map_image_list(img_file: &str, args: &Vec<String>) -> HashMap<Colour, Colour> {
-    let img = ImageReader::open(img_file).unwrap().decode().unwrap(); // FIXME: unwraps
-
-    let all_cols = extract_colours(img);
+fn map_image_list(all_cols: &ColourSet, cols: Vec<Colour>) -> HashMap<Colour, Colour> {
     let mut output_cols: HashMap<Colour, Colour> = HashMap::new();
 
-    args.iter().skip(2).for_each(|c_str| {
-        let c = decode_hex(c_str).unwrap(); // FIXME: unwraps
-        if let Some(col) = closest_colour(c, &all_cols) {
+    for c in cols {
+        if let Some(col) = closest_colour(&c, &all_cols) {
             output_cols.insert(c, col);
         }
-    });
+    }
     output_cols
 }
 
-fn map_image_image(img_file_1: &str, img_file_2: &str) -> HashMap<Colour, Colour> {
-    let img_1 = ImageReader::open(img_file_1).unwrap().decode().unwrap();
-    let img_2 = ImageReader::open(img_file_2).unwrap().decode().unwrap();
-
-    let img_1_cols = extract_colours_set(img_1);
-    let img_2_cols = extract_colours(img_2);
+fn map_image_image(img_1_cols: &ColourSet, img_2: DynamicImage) -> HashMap<Colour, Colour> {
+    let img_2_cols = extract_colours_set(img_2);
 
     let mut output_cols: HashMap<Colour, Colour> = HashMap::new();
-    img_1_cols.iter().for_each(|c| {
-        if let Some(col) = closest_colour(*c, &img_2_cols) {
+    img_2_cols.iter().for_each(|c| {
+        if let Some(col) = closest_colour(c, &img_1_cols) {
             output_cols.insert(*c, col);
         }
     });
@@ -103,22 +99,29 @@ fn map_image_image(img_file_1: &str, img_file_2: &str) -> HashMap<Colour, Colour
     output_cols
 }
 
+fn encode_hex(c: &Colour) -> String {
+    format!("#{:02x}{:02x}{:02x}", c[0], c[1], c[2])
+}
+
+fn pretty_print(h: HashMap<Colour, Colour>) {
+    h.iter()
+        .for_each(|(k, v)| println!("{}: {}", encode_hex(k), encode_hex(v)))
+}
+
 fn main() {
     dotenv().ok();
     pretty_env_logger::init();
+    let cli = Cli::parse();
 
-    // TODO (high priority): proper arg parsing
-    let args: Vec<String> = env::args().collect();
-    if args.len() > 2 {
-        // list of colours and image
-        // let out_cols = map_image_list(&args[1], &args);
-        let out_cols = map_image_image(&args[1], &args[2]);
+    let set = extract_colours(cli.image);
 
-        out_cols.iter().for_each(|(k, v)| {
-            println!(
-                "({}, {}, {}): ({}, {}, {})",
-                k[0], k[1], k[2], v[0], v[1], v[2]
-            )
-        })
+    if let Some(cs) = cli.colours {
+        let colours = map_image_list(&set, cs);
+        pretty_print(colours);
+    }
+
+    if let Some(i) = cli.second_image {
+        let colours = map_image_image(&set, i);
+        pretty_print(colours);
     }
 }