Remlins

Gremlins in folk lore is wide and varied, to try and pin a specific look down, would not be exactly easy. Though it is common that they are rather small, I'd say about 2' to 3' or so.

In my work they are between two to three feet tall, longer arms than the rest of their body, big ears, very low IQ and often seen as pawns. They are afraid of crowds in general and prefer to hunt in packs so that they may vastly out number the target. 

As for their color scheme, I would say dark green and gray. They use their claws and teeth mostly to attack.

Often seen as scavengers, or having bandit like qualities, again, they are rather varied, though normally depicted with small tusks(or not) Longer noses and a rather violent nature. They are also see as a more weak like enemy.

In my work, they are from 3-4 feet tall, arms are a bit longer than the rest of the body. They come in dark green, Gray and in rare cases, dark yellow. These goblins normally scavenge for food and will attack small groups of mortals (Humans, elves, dwarves, Ect) if they can ensure victory. Goblins. Goblins will take gold, food and weapons from mortals when possible and can use lower quality or lighter gear. Though their IQ is low, they are not without their tricks. Some can use base level magic as well as very primitive bows and arrows.

SIDE NOTE:
If you are thinking of Goblin Slayer Goblins.... No... And if you don't know what I mean by this, that is probably for the best. I'll just leave it at that. 

FOR REASONS!

Again, it varies on who is writing about them. They may have a Cave Troll, or the trolls could sing, especially if you just look up trolls. They vary from bit to small, powerful and weak. Though if you look at Nordic folklore, they are rather big and ugly, with warts or boils.... or some odd bumps growing on the body.

However, I have always though of Trolls being human sized, so that is how I will describe them in my word. from 5 feet to 6 feet or so, plus you have things like the Wartusk trolls which are a bit bigger than normal trolls and longer tusks.

Anyway, they come in the colors of Dark green, dark yellow, gray and on rare occasions dark red. They tend to have 1-3 inch tusks growing from the sides of their mouths, unless they break. They can use more advanced weapons, and have a rather violent temperament, they are also the most territorial of the Remlins. There is a small chance to bargain with them to keep your life, but they might just kill and eat you instead. They are about as smart as an average human, just with out the humanity.

Though in many folklore, Ogres and Trolls are about the same size, described as an ugly large man-like being, but yet again, the way they act and look varies on writer.

I personally have always viewed the Ogres to be larger than Trolls, but also more stupid. They are a bit smarter than Goblins, but that isn't saying much.
They come in the colors of dark green, gray and a lime like color. It is very rare to see a black ogre, and if you do, please, just run.
Ogres do not use advanced weapons like the Trolls do, but they do wear heavy armor from time to time. 

Ogres can be amused, and this can give one an opening to flee, they are territorial, but do not see humans as a big threat, so this gives them the leeway to be a touch more docile, unless they are being lead by a pack. Though it is still highly advised to stay away from Ogres when possible.