So I just noticed that the three "Pacts" in Modern Masters 2013 all have a weird coloured dot next to their type. What's up with that? I couldn't find anything on that anywhere O_°

Summoner's Pact
Slaughter Pact
Pact of Negation

It's a color indicator. You can also find them on the transformed side of dual-faced cards. They're used to give the cards color, since their mana cost would otherwise make them colorless.
I think it was introduced in Innistrad, although cards with colors contrary to their mana cost have existed since Kobolds of Kher Keep.

It was indeed introduced in Innistrad, and cards that used to say things like "This card is blue" or "This card is all colors" have been updated in the Oracle database such that those cards have color indicators of the appropriate color(s). Devoid doesn't use color indicators because colorless is not a color.

Honestly I wish they had reprinted Transguild Courier in RTR block just so we could see what a five-color color indicator looked like.

Well, the reason Devoid doesn't use color indicators isn't due to any technical reason. It's an issue of finding a symbol to represent the lack of color in a way that players would recognize and understand. Taking a card with no Blue characteristics and making it obviously Blue is much easier than taking something with Blue characteristics and making it obviously not Blue.

"Colorless is not a color" has been a stance of WotC for well over a decade now. When they had Radiance on cards, they didn't interact with masses of colorless creatures because: "colorless is not a color." They will never bother looking for a way to make a colorless color indicator because the moment they do that they'll have to rewrite an entire history of rules to define colorless as a color. Which would then functionally change the way many cards work. Don't kid yourself with this "It's just hard to make something with {color} characteristics obviously colorless" rationalization. Making a color indicator for colorless would cause a complete overhaul on the rules, and that's a headache nobody wants to undertake, nor would such an overhaul be kind to players. It'd probably cause a fallout more horrendous than the one that was supposedly caused by the introduction of the Tribal card type. EDIT: Which, by the way, I think is just a bunch of people who left over Mythic Rare rarity and hybrid mana getting more usage who just don't want to look stupid since they (Mythics and hybrid-usage) ended up becoming big hits.

If they put color indicators on all the cards, then a grey or clear color indicator would work to show a lack of colors.

I'm not saying it wouldn't be clear that there was a lack of color. I'm saying they're not going to do it because somebody would be able to spin it as "color indicators indicate color, so a colorless color indicator means colorless is a color." And with them having a very strong/stubborn stance of "colorless is not a color," they're not going to risk making colorless color indicators. For the aforementioned (and admittedly paranoid sounding) reasons.

there's no such "spin" in the rules. the rules are what they say they are. if they make an indicator for a lack of colors and say that it doesn't meant that colorless is a color, then anyone who says otherwise isn't "spinning", they're wrong. there's no risk, because they can just say "nuh-uh" and they'll be objectively right.

Probably just a gold or yellow dot.

~SE++

204.2. An object with a color indicator is each color denoted by that color indicator.

Rules are rules. A colorless color indicator makes colorless a color.

I feel like I've had this argument before, shortly after color indicators became a thing and in response to somebody asking why Ghostfire didn't have a colorless color indicator.

Ah I see Thanks for the clarification! Although I fail to see why the cards colour isn't enough. Like, the literal obvious colour that the card has anyway.

Your argument hinges on a specific definition of "color". I suspect that rule refers to the five colors of magic and that any other color than white blue black red or green wouldn't have an impact on the rules. Unless I'm wrong, a grey or clear color indicator would not make a creature grey or clear since those are not "colors" in the Magical sense.

If you really want to play with words, then an object with a colorless color indicator is still colorless because colorless isn't it a color, therefore the colorless indicator doesn't denote any color.

This is why:

202.2. An object is the color or colors of the mana symbols in its mana cost, regardless of the color of
its frame.

Before color indicators, rules text was the only thing that could go against this rule. Since the Pacts have as a cost, they would be colorless due to 202.2b:

202.2b Objects with no colored mana symbols in their mana costs are colorless

Pre-color indicators, the Pacts had "{this} is white" or "{this} is red" rules text. Post-color indicators, it's a dot to the left of the typeline that determines the color of the card. This rule was added to the rules to cover for that:

202.2e An object may have a color indicator printed to the left of the type line. That object is each
color denoted by that color indicator. (See rule 204.)

Mostly as a redundancy for the Color Indicator section.

Going full pedant, white and black aren't colors and so shouldn't be covered by this rule.

If colorless isn't a color, then how can a color indicator indicate that a card is colorless? It can't.

Therefore, if a colorless color indicator exists, then colorless is a color for the color indicator to indicate.
Hence, colorless color indicators will never exist. Because colorless isn't a color.

@TP: White and black are colors... They just aren't spectral colors (Wikipedia calls them achromatic colors). It turns out defining color by light spectra breaks down rapidly (e.g. Magenta), so we are left with perception of rod/cone stimuli, in which white and black certainly exist.

By that logic Edacade, my fuel indicator could never read empty, so I should stop wasting all my money at gas stations.

How exactly are you applying "that logic" to your fuel indicator? You might be doing it wrong.

In that case fuel is a quantity, so you can say 0 units of fuel is a number of units of fuel. But yes, you have demonstrated that the basic logic of the argument is flawed.
A better example would be if somebody asks what your religion is, and you say you aren't religious. That's still a valid answer even though it is not a religion.
In the same way, a color indicator that shows no color can denote colorless.