First, the browns:
On the top row are the old plastic models. First is the chestnut #5702, this run is usually more orange-toned with a mane and tail slightly darker than the body colour. In the middle, bay #5700 is a sandy bay, darker and more brown-toned than #5702, with dark brown to black mane and tail, the legs are most often body-coloured instead of dark. Lastly is the foal from the Stable Set. This model is usually quite a bit darker and more plum-toned than #5700 with black to dark brown mane and tail.
On the bottom are the new plastic models. To the left is the foal from the 1997-99 Arabian and Foal set. This model is a light yellow-tan chestnut, sometimes with a bald-ish face and usually with vague socks. The other foal is from the 2002-05 Fun Foals set. More dun than chestnut, this run is darker, more brown-toned, and sports darker points, distinct socks and a dorsal stripe.
The other look alikes are the two black releases.
Most obviously, #5701 on the left is old plastic, while the 2001-05 Horspital foal on the right is new plastic. #5701 is a duller black which ranges from glossy to matte in finish and can sport a range of airbrushed socks. The Horspital foal is a deeper colour, usually matte or satin, and is consistently marked with a star and four stenciled socks.