Place egg yolks in a small bowl and crush until no large chunks remain. Add mayonnaise, mustard, and salt; stir until well combined.

Decide how many different varieties of Halloween deviled eggs to make. If making all 4 varieties, divide yolk filling into 4 small bowls and set aside 6 whites for each variety.

For Eyeball Deviled Eggs: Stir purple food coloring into egg yolk mixture until desired color is achieved. Fill egg whites with purple yolk mixture. Top each egg with a slice of olive resembling an eyeball.

For Pumpkin Deviled Eggs: Stir orange food coloring into egg yolk mixture until desired color is achieved. Fill egg whites with orange yolk mixture. Use a toothpick to make lines resembling pumpkin creases and garnish with parsley for the "stem.".

For Spooky Green Deviled Eggs: Stir green food coloring into egg yolk mixture until desired color is achieved. Fill egg whites with green yolk mixture. Sprinkle eggs with black sesame seeds.

For Spider Deviled Eggs: Fill egg whites with egg yolk mixture (no food coloring needed). Cut black olives into “spider” parts and arrange on top of eggs.