There are 2 things in an image, Luminance and Chrominance. Luminance give brightness (intensity) information and human eye is very sensitive to this information. Chrominance give color information and a small variation in it is not much detectable by eye.
To find chrominance in image, color space like Lab, HSV YCbCr etc can be used where L , V and Y give luminance respectively while other terms give information of chrominance.