Standard Material: 
It is like the Default Material but can have assigned an effect.

Grayscale Material: 
Applies a simple shader which displays the colors of the sprites as grayscale values.

The Amount slider (introduced in Better UI 2.3) allows to interpolate between color and grayscale.

Hue Saturation Brightness Material: 
Allows to shift the colors of the sprites (hue), make them intense or gray (saturation) and darker or brighter (brightness). This is a very powerful shader which is shared between all Images using it. However, the values can be different on each instance. This is possible because of a trick and is much better for performance if you have a lot of images with this shader.

Not supported for Images with Filled mode.

Color Overlay Material: 
The given color is not used to mix it with the sprite color but to overlay it. A value defines the opacity of the color (A value of 0 shows the original image, a value of 1 shows the color only).

Not supported for Images with Filled mode.