Overo pronounced "oh vair' oh"

GENERALLY….The white does not cross over the back between the withers and tail.  The overo will have a solid strip of color running down its back from the withers to the tail that is from an inch wide to several feet wide. 

This strip may not appear solid at times, if the horse is a medicine hat or sabino  (pronounced "sah-bee-no") marked overo.  It may look like frosty speckling down the spine.  Do not confuse this coloration with the definite dorsal stripe on a buckskin.  An overo cannot produce a tobiano or tovero foal unless one of the parents has tobiano or tovero genetics in its background. 

This is a classically marked overo.  Bald face with two dark legs, and a splatter pattern of white on the body. Although it's close, no white actually crosses over the back between the withers and tail.

Tobiano pronounced "tow be yah' no"

GENERALLY….The white does cross over the back between the withers and tail.  The face is usually dark with star, strip and snip.  The tail and mane are often multi colored and the legs are white. An overo that has been bred to a tobiano can produce an overo, a tobiano, a tovero or a solid breeding stock paint that doesn't have enough white to be included in the regular registry.

Breeding a tobiano to a tobiano will produce a tobiano foal.  A tobiano sire and tobiano dam cannot produce an overo foal unless there are overo or quarter horse genes in the background of the pedigree.

This is a classically marked tobiano.  Dark head with a white star, four white legs, a shield marking over the tail area, and the white crosses over the back between the withers and tail.

Tovero pronounced "tow vair' oh"

GENERALLY….The white does not usually cross over the back between the withers and tail but it can cross over, as these horses combine the characteristics of both the overo and tobiano horses.  Many tovero horses have bald faces.  An overo that has been bred to a tobiano can produce an overo, a tobiano, a tovero or a solid breeding stock paint who doesn't have enough white for the regular registry.  It gets pretty confusing when the types are mixed and many of the more exotic coat patterns are a mixture of many of these types of crosses.

This is a classically marked minimum colored overo.

Blaze face with two dark legs, and a smaller splatter pattern of white on the body.   Minimum colored overos can be solid colored with excessive white that passes behind an imaginary line you draw between the corner of their ear and mouth.  These horses may only have one small spot underneath their belly or between the cheeks of their buttocks that is only two inches in length, as long as the underlying skin has a contrasting spot of pink skin.

This is an example of a classically marked "frame" overo.

Even though the white comes close to crossing over the horse's back between the withers and tail, it does not cross.  The horse's profile is  "picture framed" around the edges with dark hair.

Note that the white DOES NOT cross over the back between the withers and tail.
 

Here are two more examples of a classically marked tobiano.

Dark head with a white star, strip and snip four white legs, a shield marking over the tail area.  Dark head with a white star, strip and snip four white legs, a shield marking over the tail area and in the chest, multi-colored tail, and the white crosses over the back.

Note that the white DOES cross over the back between the withers and tail.

Homozygous Tobiano

The white crosses over the back between the withers and tail.  Characteristic "ink" spots and "paw print" markings on shoulder and hip.

Summary...It's not the amount of white on a Paint horse that determines its "type…...it's whether the white crosses over the horse's back, between the withers and the tail and, in the case of the cross-bred Tovero, that generally has one Tobiano parent and one Overo parent, it's how those markings sit right in the middle between both "types" that gives it its classification.  It is impossible to raise an overo out of two tobiano parents and vice versa.  You won't get a tobiano by breeding an overo to a quarter horse. 

Overo horses have no color gene in their DNA makeup.  Tobiano horses do have a color gene and blood markers that further designate whether it is: "homozygous"  or "heterozygous"  or "informative" or "uninformative".