Four very convincing theories about why the royals always wear the same colour head-to-toe.

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This week, Meghan Markle left her home twice in outfits styled head-to-toe in a single colour.

That colour happens to beige, but that’s neither here nor there. The real feat is managing to match your entire outfit with items in the exact shade. We’re talking more than just matching nail polish or red lippie with a red dress.

On Thursday, the Duchess of Sussex visited The National Theatre in London wearing a neutral Brandon Maxwell V-neck dress and blazer paired with a neutral clutch with gold detailing and the neutral suede Aquazzura ‘Matilde’ Crisscross heels she wore for her engagement photo call with Prince Harry.

The week before it was a sandy beige knit dress with a sandy knit trench coat and sandy suede slingback heels.

Upon further investigation, it's clear this whole tonal colour blocking is somewhat of a royal fashion phenomenon.

The Duchess of Cambridge Kate Middleton was doing this waaaaaaay before Meghan joined the Kensington Palace group chat; green was her colour of choice this week, pairing a long-sleeve green dress with a green belt, clutch and suede heels (why do royal love suede so munch?). Over Christmas, both duchesses walked side-by-side, Meghan wearing all-navy and Kate in all-burgundy.

At Prince Louis' christening, the entire motley crew of royals from Pippa Middleton to Camilla Parker Bowels wore head-to-toe outfits in a single colour.

Which got us thinking... why? Why the need to wear perfectly matched outfits all the time, hey?

Royal dress codes, especially for royal women, are widely documented.

No bright nail polish, only a nude or pale pink like Essie's Ballet Slippers. No open-toed shoes. No wedges in front of the queen. No taking your coat off in public. No dresses or skirt above the knee. Legs must be modestly covered in skin-coloured tights at all times.

Hence, one could conclude there is also a rule about having to match your dress to your shoes to your bag to your hat at all times.

Here are four somewhat reasonable theories as to why the likes of Kate Middleton and Meghan Markle are always dressed in one colour from head-to-toe.

Before we get into this very important investigation, here's a style recap of Meghan Markle's fashion before and after becoming a royal. Post continues after video.

1. It's fashion, darling.

Tonal colour blocking or tonal dressing isn't a new trend, nor is it something only royals can do.

The introvert's version of colour blocking, tonal dressing involves piecing together items in the exact same shade (or same colour in lighter and darker shades) for an elongated, no-fuss silhouette that's easy to pull together when you're in a rush.

It's also just really fun. Perhaps Meghan might stray from forest green and try it in leopard one of these days.

2. The slideshow dressing theory.

So many...of the same colors and styles! My royal tour recap is up in Stories! ????⬆️???? Can you see the progression here? ???? Spot the trends? ???? Crew necks and dark skinnies and tied belts, oh my! ???? Share your Thoughts™️ below ????⬇️????

A post shared by Elizabeth Holmes (@eholmes) on Nov 5, 2018 at 7:09am PST

This is a genuine theory from journalist and writer Elizabeth Holmes, who documents the royals' style on her blog So Many Thoughts, and Instagram account.

Elizabeth calls her theory slideshow dressing, and it makes a heck of a lot of sense.

"Meghan is building her brand, and crafting a signature style, very deliberately. It began shortly after the wedding, when Meghan wore a string of blush dresses that were all variations on one another. Then she moved to boat-neck styles, very reminiscent of her Givenchy wedding dress, but in a range of colours. On the royal tour, she favoured solid-colour dresses with crew necks, nipped waists and full skirts," she wrote.

"The result is often a slideshow — literally! — compiled by the websites and fan accounts that follow royal style. It makes it very easy, and visually appealing, for editors to draw comparisons and make collages of past looks. And for the public to get a sense of her personal style."

3. Dressing in one colour saves time.

RE above when we were discussing tonal dressing, pairing items of the same colour/in the same colour family shaves time off getting your outfit ready the night before, or grabbing something in a hurry in the morning.

Even more so if you organise your wardrobe by colour.

Look, neither Meghan or Kate are likely laying out their own clothes the night before. But at least when your boss texts you at 5am to say they feel like wearing beige today instead of brown like they said yesterday, the royal style assistants can quickly locate multiple outfit options that fit the brief.

Side note - here's some more of the extreme royal dress codes explained, post continues after audio.

4. It's a respect thing.

This theory does have some strong evidence behind it.

Starting back when Queen Elizabeth II first started travelling overseas on royal duties, using your clothing to honour the country you're visiting is a reasonably simple way to make the people of the Commonwealth feel seen. That the Queen knows who we are, and we're not just a bunch of ants in her Commonwealth kingdom.

This is often done with designers - both Meghan and Kate wore Aussie designers on their respective Australian trips - but it can also be done with colours that have a cultural significance.

Most recently, Kate wore an all green and navy outfit featuring tartan while visiting Dundee in Scotland.

No doubt, the ensemble wasn't pulled together by chance - first, the appropriate tartan to honour the people and location would have been selected, and the rest of the outfit styled in tones to match from there.

4. The Queen does it.

As we all know, the Queen is the boss of all these kinds of royal rules and protocols.

She also never leaves the house without wearing some kind of matching bright ensemble, complete with coordinated bags and hats.

While it's widely reported the Queen wears bright colours so commoners her fans can spot her in a crowd, dressing head-to-toe in neon green and canary yellow appears to be a personal style choice...

... Which means it's a style rule for all the royals.

And you know when Liz wants something, y'all best fall in line.