On colorless decks

Colorless decks have three big inherent weaknesses:

• Most cards have a big upfront investment cost and are expensive for their effect.
• Very few cards exist that can efficiently interact the the opponents.
• We have little protection against and are very susceptible to the interactive cards the opponents wield.

It's important to select cards that can generate value the moment they are played.
This is even more important if their initial investment cost is high (e.g. Citanul Flute, Myr Turbine, Kozilek, Butcher of Truth, ...).

There are in practice only 2 good (but expensive) mass removal spells in our "color": Ugin, the Spirit Dragon and All is Dust.
Our single target removal options are generally terrible. Ugin, the Ineffable is probably the best one, but from there it goes downhill fast.

The only 'defense' against mass artifact removal is to keep our hand full of cards.
It's important to include enough sources of direct and indirect card advantage.
Even then, cards like Null Rod, Stony Silence, Collector Ouphe or even Titania's Song can cripple the deck to the point where winning becomes almost impossible.

Colorless decks have one big advantage:

• It's impossible to get color screwed, we can play all the efficient colorless mana ramp and as many utility lands as we want.
  This means we can often keep pace with the green big mana decks and can try to overpower the other players.

Combo

Combinations are the most effective way to win with a Karn, Silver Golem deck.

Combo cards were selected for their usefulness outside of the combo and their mana cost.
Cards like Krark-Clan Ironworks, Rings of Brighthearth, Staff of Domination and Salvaging Station can at least provide some value outside of their combo role
Most combo turns end with us playing Walking Ballista for an arbitrary amount of mana.
Other potential combo finishers are Mimic Vat, Myr Turbine and Retrofitter Foundry.

Infinite combo's:
Karn, Silver Golem + Voltaic Construct + Basalt Monolith/Everflowing Chalice/Grim Monolith/Mana Vault/Thran Dynamo
Karn, Silver Golem + Salvaging Station + Mana Crypt
Karn, Silver Golem + Salvaging Station + Sol Ring/Mana Vault + Skullclamp
Rings of Brighthearth + Basalt Monolith
Metalworker + Staff of Domination
Sensei's Divining Top + Cloud Key/Ugin, the Ineffable + Mystic Forge

And many, many more variants.

On Wastes

Basic lands are often undervalued.

They allow us to use cards like Burnished Hart, Solemn Simulacrum, Wayfarer's Bauble, Thaumatic Compass, Extraplanar Lens, Thawing Glaciers and Endless Atlas.
These cards provide card advantage or allow us to ramp in ways that are usually not available to artifact decks.

The only non-basic lands that made the cut are sources of card advantage, land destruction or lands that have very powerful effects like Blast Zone or Inventors' Fair.