Q:

Are multicolor decks in any color combination viable in modern Magic?

I've just recently regained interest in MtG and was overwhelmed by the quantity of new collections and the amount of cards.
I stopped playing MtG when the Fallen Empires expansion was just out. 
One of the things that pops up immediately when browsing the cards (aside from the numerous new mechanisms) is that there's a lot of multicolor cards.
In the old days, building a deck of more then one color was very hard, and in most cases it involved playing at least green because of Birds of Paradise (BoP). In terms of tactics this was very limiting, since green used to be the way for cheap creatures.
Now it seems pretty much every combination of color is a viable tactic. Is that true? Or, are there still some colors that work better together?
One other thing; I used to stick very stubbornly to a 60 cards maximum deck size, with two copies of most cards (except lands), and four copies for some very useful cards (one example was Giant Growth when playing green). Do cards now exist that help the probability of running the deck fluidly, rendering obsolete my stubornness for requiring no more than a 60 card deck?

A:

Running 60 is still the best plan. You have the highest likelihood of drawing into your best cards when you have fewer cards.
As far as seeing more multi-color, right now we're in a multi-color block. Retrun to Ravnica (RTR)and Gatecrash (GTC) each feature five of the ten Ravnica guilds.
Right now in current standard are the ten 'shocklands', lands that come into play tap unless you pay two life and can tap for two colors (in RTR and GTC), for example Blood Crypt; a cycle in M13 of allied dual lands which come into play tapped unless you control a land type that it taps for, for example Glacial Fortress, and a cycle of enemy dual lands which do the same from Innistrad, for example Isolated Chapel.
Dual lands are the primary source of mana fixing. RTR and GTC also released gates, which are dual lands at common that always enter the battlefield tapped. These are typically considered too slow for standard constructed play.
Green is still considered the fixer color, granted access to tools like Caravan Vigil, Arbor Elf, Avacyn's Pilgrim, and Farseek, Mulch, Ranger's Path, Seek the Horizon, or if you have insane mana already like perhaps in a Commander game, Boundless Realms.
Currently Llanowar Elves and Birds of Paradise have rotated out of standard. They aren't usually out for long, so we stand a chance to see them in 2014, but honestly with all the fixing in right now, they aren't as necessary.
As to your question of "pretty much every combination of color is a viable tactic" you could make that argument. Each two color combination has a Ravnica guild, and half of the three color combinations have an Alara shard. Each of the Ravnica guilds is seemingly viable in standard, although many of them do have a third splash color. There are fewer monocolor top decks right now just because the set is so focused on multicolor decks.
Other formats have different archetypes. There is no single Commander deck that is winning everyone's hearts and minds. Jund (and to some extent, Storm) is completely dominating Modern to the extent that the DCI just banned two cards, one core to each deck, to level the playing field a little. Legacy and Vintage I don't follow enough to really comment on, but I'll say as far as I know there are no dominating color patterns there.

A:

As I said in a comment, I would dispute the very premise of this post — not only have there been tournament-viable multicolor decks for as long as there have been Magic tournaments, but I would say that over Magic's history it's more often been the case that a multicolored deck is (one of) the best in the format than that it hasn't.  (The three exceptions that come to mind are the Buehler-style mono-Blue control decks in post-bannings Tempest/Urza standard, the mono-Black 'Napster' control deck that won the world championship a couple of years later, and various flavors of mono-red decks)
One deckbuilding principle that's been fairly consistent throughout Magic's history is the 'power splash': stretching your mana base a little bit to include one or two cards that provide an effect you couldn't get otherwise.  Brian Weissman's inclusion of Demonic Tutor and Mind Twist into his otherwise pure Blue/White control deck is a good example of this; modern versions include a very similar idea, UW control decks splashing just a little Black to be able to play Ultimate Price to destroy early creatures and (most importantly) activate Nephalia Drownyard as the deck's primary (uncounterable and hard-to-destroy) win condition.
As far as the state of tournament Magic right now, one good resource for checking out what decks are doing well (at least in a particular metagame) are the current winning decks on Magic Online, which you can find in a sidebar on the "What's Happening" page on Wizards' Magic Online site.  From a cursory examination of that you can find that mono-Red decks are still doing well, but so are White/Green/Black reanimator decks, and Blue/White control decks (often splashing a little bit of Black), and Red/Green/Black midrange decks with lots of value creatures, and White/Green/Blue decks, either pure control or with lots of acceleration into larger creatures, and White/Green/Red decks also on the big-guys plan, and Green beatdown decks splashing a little Black, and a Green/Blue/White 'enchantments' deck, and... well, you get the idea.

