Stan Goldberg was the lead colorist for Marvel in the formative years of the Silver Age and was primarily responsible for choosing the color schemes for virtually all the first generation heroes and villains. Interestingly (and please note this is not an original observation), he chose a formula for differentiating the good guys from the bad guys: heroes wore primary colors (blue, red, yellow), villains wore costumes predominated by secondary colors (green, orange, purple, plus brown and/or grey). Need proof? Here’s a breakdown:

Heroes

Ant-Man/Giant-Man – red, blue
Captain America – blue, red
Daredevil – yellow, red (first costume), red (second costume)
Dr. Strange – blue, red, yellow
Human Torch – red, yellow (aflame), blue (as Johnny)
Iron Man – yellow (original armor), red, yellow (second armor)
Mr. Fantastic and Invisible Girl – blue
Scarlet Witch – red
Spider-Man – red, blue
Thor – blue, red, yellow
Wasp – red, blue
X-Men – blue, yellow

Exceptions

Hawkeye – purple, blue
Hulk – green, purple
Quicksilver – green
Thing – orange, blue

Note that these four are all either reformed villains or monsters.

Villains

Abomination – green
Absorbing Man – grey
Aggamon – green, red
Asbestos Man – purple, blue
Awesome Android – grey
Baron Mordo – purple (first appearance), green (subsequent appearances)
Baron Zemo – purple, red
Beetle – purple, green
Cannonball – brown, grey
Clown – brown, orange, yellow
Cobra – green, purple
Diablo – purple, green
Dormammu – green (first appearance)
Dr. Doom – green, grey
Dr. Octopus – white (first appearance), purple (second through fourth appearances), green, orange (costume)
Dragon Man – grey
Eel – purple (first and second appearances), green (third appearance on)
Electro – green, yellow
Enchantress – green, yellow
Executioner – purple, blue
Great Gambonnos – purple
Green Goblin – green, purple
Grey Gargoyle - grey, blue
Hate-Monger – purple, grey, brown
Human Top – green
Immortus – purple, green
Impossible Man – green, purple
Infant Terrible – green purple
Juggernaut – brown, red
Kang – purple, blue
Kraven the Hunter – brown, orange, green
Leader – green, purple
Leap Frog – green
Living Eraser – green, purple, grey
Lizard – green, purple
Loki – green, yellow
Lucifer – purple, red
Mad Thinker – green
Magneto – purple, red
Mandarin – green, purple
Masked Marauder – purple
Mastermind – brown
Matador – green, orange
Melter – green, blue, brown
Mole Man – green
Molecule Man – green, purple
Mr. Fear – purple, blue, red
Mr. Hyde – green
Mysterio – green, purple
Nightmare – grey (first appearance), purple (second appearance), green (subsequent appearances)
Owl – green, purple
Paste-Pot Pete/Trapster – green, purple (as Pete), purple, blue (as Trapster)
Plantman – green
Plunderer – purple, white
Porcupine – brown, blue
Power Man – brown, orange
Princess Python – green
Purple Man – purple (duh)
Radioactive Man – green, purple
Rama-Tut – green, yellow
Red Ghost – green
Rhino - grey
Ringmaster – purple, green
Sandman – green, grey (solo costume), purple, grey (Frightful Four costume)
Scarecrow – green, orange
Scorpion - green
Sentinels – purple, blue
Shocker – brown, yellow
Skrulls – green, purple
Space Phantom – purple
Stilt Man – grey
Swordsman – purple, blue, yellow
Tiboro – purple green
Toad – orange, grey
Unicorn – green, orange
Vanisher – green
Vulture – green
Wizard – green (original costume), purple, blue (Frightful Four costume)
Xandu – green

Exceptions

Crimson Dynamo – red
Gladiator – yellow, blue
Molten Man - yellow
Unus - red

Note that these four villains’ color schemes contrast with that of their primary enemies.

There is no such pattern discenible in the DC comics of the same period. I don’t know what it all means, but I find it interesting.

Cei-U!
I summon the Crayolas!