Colorization is the process of introducing color to grayscale digital images. In these images, each pixel has a scalar value representing its intensity. However, the pixels of color images contain more complex, three-dimensional information. Depending on a color model, the pixel attributes correspond to a three-value color representation. For the purpose of grayscale image colorization, we strongly advise the use of color representation, which allows easy luminance extraction. This makes the colorization simpler-we only need to estimate two chrominance channels, while the luminance of the colorized image remains unchanged.Despite plenty of color models, we cannot directly transform a one-dimensional grayscale value to a three-dimensional color space. We only get the luminance of the colorized image, while the two remaining color channels are still unknown. Therefore, the colorization may be performed in many different ways, achieving various outcomes. Additionally, it is difficult to objectively assess the final colorization results. We can compare the colorized version using standard image quality measures only if we have the color version of the image. The colorization of grayscale images without a reference has to be evaluated mainly subjectively.In general, there are two main types of colorization frameworks. The first one is the semiautomatic approach. In such a range of methods, a user has to indicate the color hints in form of scribbles within the grayscale image. The scribbles enable the algorithms deciding which colors have to be put in corresponding parts of the image. The colorization procedure in the semiautomatic approach may be visualized as color spilling over the image areas starting from the indicated scribbles.The automatic approach employs a color image that resembles the colorized grayscale picture. The algorithms rely on the similarities between the luminance B. Smolka ( ) · A. Popowicz