Goblins:
BX "Their skin is a pale earthy color, such as chalky tan or livid gray. Their eyes are red, and glow when there is little light."
AD&D "Goblins range from yellow through dull orange to brick red in skin color. Their eyes are reddish to lemon yellow."
2e "Their skin colors range From yellow through any shade of orange to a deep red. Usually a single tribe has members all of about the same color skin. Their eyes vary from bright red to a gleaming lemon yellow."
Green is the "classic" goblin color in a lot of of media, and lots of people just translate that to their DnD art without thinking. Magic the Gathering cards depicted them as green. Same goes for Warcraft. Warhammer would be another example, and Pathfinder's Goblins are green. Dragonlance goblins varied from white, to grey, to red, to brown, to yellow etc. It was a clan thing.
Green orcs were cemented into RPGs by Games Workshop (Warhammer) when they started selling their orc models (GW started out as a D&D mini company with the Warhammer rules existing just to get people to buy more minis than they needed). One of their artists thought that green orcs looked funny and distinct. Then Warcraft copied it and took the idea of green orcs to the mainstream. Before GW, no orcs were green - Tolkien orcs were brown and grey, most D&D setting orcs were also grey, etc.
Though the above only covers the "monster manuals". Dozes of source books over the years have had orcs and goblins of all skin colors. Plus you need to add in all the environmental ones like the "swamp orcs" and "ice goblins" that were green and white.
So, really....all colors.
