No no, armor works best against high rate of fire weapons. 1 armor = 5 damage reduction, but you only lose 1 armor each time you're hit. So if you have 50 armor, it would be best if you were hit 50 times for 250 damage reduction.
Source: https://www.reddit.com/r/Overwatch/comments/3doh28/difference_between_health_and_armor/
In plain terms, white health is better than armor (you'd rather have 250 health than 250 damage reduction spread over 50 hits). The armor mechanic just gives tanks better survivability against weapons with high rate of fire.
[Edit] More info:
Shield (blue): Consumed first. Regenerates after you stop taking damage. Cannot be healed.
Armor (yellow): Consumed second. Reduces damage from each hit by 5. Does not regenerate. Can be healed if it is "natural" armor. Natural armor is armor that characters have built into their health (i.e. they spawn with it). Bastion, Reinhardt, and Winston all have natural armor. Armor from Torbjorn's armor pack cannot be healed and is lost when consumed.
Health (white): Consumed last. Does not regenerate. Can be healed.
Source: https://www.reddit.com/r/Overwatch/comments/3r6834/questions_about_armor/
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