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

Decide on the number of different varieties of Halloween deviled eggs to make. If making all 4 varieties, divide the yolk filling into 4 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.

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 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 with black sesame seeds.

For Spider Deviled Eggs: Fill egg whites with egg yolk mixture without adding food coloring. Cut black olives to create 'spider' parts and arrange on top of eggs.