Why don't Americans write devor instead of devour to be consistent with the pervasiveness of using variations such as color and armor?
No it wouldn't, because devour doesn't rhyme with colour/color or armour/armor. It does rhyme with hour.
In other words, the ending -our is only respelled as -or when it represents an unstressed, r-colored schwa [ɚ]. The stressed diphthongs in hour, devour, flour, etc. retain their original spelling.
English is a language influenced by Latin. The word colour/color comes from "color, -oris" whereas armour/armor comes from "armatura, -ae".
"-or"/"-our" are all Latin based noun endings that can be found in other European languages too, for instance in French or German.
"devour" on the other hand is a verb and verb endings follow different rules. "devour" comes from "devorare". The ending "-are" got dropped as centuries passed. Something similar happened to initiare -> initate, abdicare -> abdicate and many other similar words.
"Color" and "armor" all end with the same sound as the word "or".
The ending of "devour" sounds the same as the word "our".
If we changed it to "devor" it would change the sound of the word. We changed the spelling because our way makes more pronunciation sense. You don't go around saying "col-hour" do you? Because that's what the British spelling looks like.
The real question should be: Why do the Brits use letters that have the wrong sound?
Why do they spell "practice" like "practise". It looks like it should be pronounced "prak-tize" when spelled that way. "Organize" vs "organise", "civilized" vs "civilised", etc. I could go on and on. The fact of the matter is that American English is more modernized (probably because we're a newer country.)
P.S. I'm not trying to be rude, but the British spellings look archaic to me.