Two shades of a Grey scale when placed side by side creates a contrast
There are 3 main contrasts :

a. High Contrast      b. Middle Contrast      c. Low Contrast

High Contrast - When one shade from High Key and one shade from Low Key are used in one color combination then it is called 'High Contrast'.

Middle Contrast - When one shade from Middle Key is used with another shade by leaving two stages from High or Low Key in one color combination, it is called as 'Middle Contrast'.

Low Contrast - When any alternate two colors in Grey scale are used in one color combination, then it creates 'Low Contrast'.

This contrast can be created in any key.
eg. High Light - Low Light or High Middle - Low Middle .

Tint : White color mixed in any color with more or less proportion then that shade is considered as a 'Tint'. It gives flatness to the picture.

Tone: Grey color mixed with any color is called as 'Tone'. Due to absence of Tint and shade there is no contrast.

Shade : Any color mixed with black yields a dark shade of that particular color. If a shade is used with Tint and Tone then it gives a more prominent look.

While making color gradation , place of a color should be fixed within the Grey Scale as shown below.

(High Light) = Yellow
(Light) = Yelloworange.Yellowgreen
(Low Light) = Orange and Green
(Medium) = Redorange.Bluegreen
(High dark) = Red and Blue
(Dark) = Redpurple and Bluepurple
(Low Dark) = Purple