I think this may become one of the bigger factors in determining 2 versus 3+ colour decks in invasion limited, specifically sealed. It has always been a factor in limited but this set brings it to the forefront. Namely, a sealed deck in masques would get you on average 13-15 cards of each colour. So a 2 colour deck would have to choose all of it's 22-24 cards from a 27-32 card pool. A splash of a third colour provides 15 more options, and if it's planned, gives several post sideboard options. Expressed differently, if every land was a 5 colour land, no one would play 2 colour decks. Most decks would be 3 or 4 (occasionaly 5) colours, because they would simply use all the best cards. Because mana colour is a factor though, one must sacrifice card quality for consistency. In masques block, this is often a tricky choice. Playing 3-4 mediocre cards and 2 or so cards that are slightly better against sneaking in possible mana problems in order to have more options and optimal cards.
Cut to invasion. Your typical sealed deck will now have several gold cards, many of them very powerful. This has 2 effects
1) you get less cards that are 1 colour only, and the gold cards between 2 specific colours wont be high on your count, so a two colour deck has less to choose from.
2) you'll be constantly tempted by possible breakers that you are 'halfway' to casting.
So while blue search and green mana cards are what make multicolour so consistent (and it is remarkably so) in Invasion, it would probably be chosen anyway. Why? That 30 card pool just became a 27 card pool, with possibly 2 or 3 gold cards that you need one more colour to use. then you have the single coloured mana spells of that colour, and all of a sudden, a third colour gives you several more options. At the least, this will give you a sideboarding plan. More often than not, though, it will lead to three colours as the extra options become too powerful to ignore.
The only case where I can see myself running a two colour invasion sealed deck is where both green and blue get shafted, and I have god red and black. The rest of the time, even if you can comfortably go with two colours, you probably can get more out of a 3 colour deck
I didn't quite get this... do you mean that blue search, green mana and all the cantrips make 3-color more viable... then all the gold and cross-color cards make it more rewarding?
sorry I was on little sleep and didnt express myself well. To sum up, going three colours is always a trade off between having more options available and having mana consistency. So some players in masques, for example, will accept the disadvantage of a smaller card pool (only 2 colours of cards) in order to get more consistent performance. This has already been discussed in many articles about limited. I pointed out that with the large number of gold cards, however, the average card pool that one can expect from only 2 colours has shrunk because of the presence of these cards. This means that it is more of a disadvantage to go 2 colour, as it limits ones options more severely. The rest of the mechanics in Invasion only serve to exacerbate this situation. So in my opinion, 3 colour is usually the best choice in Invasion because it allows the use of twice as many gold cards (most of which are pretty powerful ) as well as a full other colour.
Summary of the summary: 2 colour invasion decks have less cards to choose from than the corresponding masques decks , while 3 colour decks get access to often powerful spells, a full 3rd colour worth of options, plus several cards to bring the colours together.
You also have to factor in things like cross-color Kicker and ability activation costs, and you really miss a lot by not going for a third color.
In addition, there are a lot of single colored mana spells that are really good, the environment is much slower, some common creature removal involves the domain mechanic, and there are color consistency measures and library manipulation spells that help... all pointing to three colors minimum.
But what about in drafts? In previous drafts (with one exception...) your card pool should be about 31 to 38 playable colored cards. The one exception for me was in Prophesy draft. I drafted 41 white cards. I won the tourney with ease. Also, in sealed deck, Invasion, I played three colors. Green was my primary followed by black distruction and red splash. Technically, I had a broken deck. But I had not studyied and should have known that everyone was playing black and green. I had great blue with 8 flyiers/evasion abilities. Why did I not play Blue? The cards were poor quality (others) and my card pool for white and black(Frendly colors) was similarly limited. My biggest problem with invasion is it's extreame emphasis on color freindly decks. It would not be possible for me to secssfuly play blue green.
In Draft the concept differs somewhat because politics come into play and other people can effect your card pool, so by limiting your colours, you limit your exposure to this problem. However, in invasion, if you get the right splash cards and/or split cards, it can be worthwhile cutting off a third colour in the last pack.