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If an ability would produce one or more mana of an undefined type, it produces no mana instead.

Some abilities produce mana based on the type of mana another permanent or permanents “could produce.” The type of mana a permanent could produce at any time includes any type of mana that an ability of that permanent would produce if the ability were to resolve at that time, taking into account any applicable replacement effects in any possible order. Ignore whether any costs of the ability could or could not be paid. If that permanent wouldn’t produce any mana under these conditions, or no type of mana can be defined this way, there’s no type of mana it could produce.

To “tap [a permanent] for mana” is to activate a mana ability of that permanent that includes the {T} symbol in its activation cost. See rule 605, “Mana Abilities.”

An ability that triggers whenever a permanent “is tapped for mana” or is tapped for mana of a specified type triggers whenever such a mana ability resolves and produces mana or the specified type of mana.

A replacement effect that applies if a permanent “is tapped for mana” or tapped for mana of a specific type and/or amount modifies the mana production event while such an ability is resolving and producing mana or the specified type and/or amount of mana.

If an object’s enters-the-battlefield triggered ability or replacement effect refers to X, and the spell that became that object as it resolved had a value of X chosen for any of its costs, the value of X for that ability is the same as the value of X for that spell, although the value of X for that permanent is 0. This is an exception to rule 107.3i.

The tap symbol is {T}. The tap symbol in an activation cost means “Tap this permanent.” A permanent that’s already tapped can’t be tapped again to pay the cost. A creature’s activated ability with the tap symbol in its activation cost can’t be activated unless the creature has been under its controller’s control continuously since their most recent turn began. See rule 302.6.

The untap symbol is {Q}. The untap symbol in an activation cost means “Untap this permanent.” A permanent that’s already untapped can’t be untapped again to pay the cost. A creature’s activated ability with the untap symbol in its activation cost can’t be activated unless the creature has been under its controller’s control continuously since their most recent turn began. See rule 302.6.

You can't choose "colorless" as a color.

If Call to Arms changes controllers, it will continue to check the nontoken permanents controlled by the player chosen as it entered, even if that player isn't an opponent of its current controller.

Call to Arms's third ability is a "state trigger." Once a state trigger triggers, it won't trigger again as long as the ability is on the stack. If the ability is countered and the trigger condition is still true, it will immediately trigger again.

The game continually counts the number of nontoken permanents of each color the chosen player controls. Multicolored ones are counted for each of their colors; colorless ones are ignored. The moment the number of nontoken permanents of the chosen color that player controls is less than or equal to the number of permanents of one of the other colors that player controls, Call to Arms's second ability stops giving white creatures +1/+1 and its third ability triggers. Call to Arms will be sacrificed when the third ability resolves, even if those numbers have changed by then.

Call to Arms affects all white creatures, not just white creatures you control.