What is meant by 'combine the luminance and chrominance ' I am new with image processing. I'm trying to implement a research paper in MATLAB. I didn't understand a sentence. 
Here it is: 


  The luminance is enhanced by applying the proposed algorithm and it is
  combined with the chrominance and converted back to color image.


How can I "combine the luminance and the chrominance"? 

Paper reference: Image enhancement with the application of local and global enhancement methods for dark images, 2017 International Conference on Innovations in Electronics, Signal Processing and Communication (IESC)
Intensity of color images are defined through color spaces such as RGB, YUV, and HSV. 

RGB is an additive-synthesis primary color decomposition and it's very fundamental. 

Whereas YUV is an example of Luminance (brightness or Black &amp; White) and Chroma (color information saturation and hue) decomposition. Usually chroma consists of two components such as U &amp; V in YUV or H &amp; S as in HSV color spaces.

When luminance and chroma are combined together they create the complete definition, YUV or HSV, of the color information which can be converted into RGB. How they are combined depends on the chosen color space and its conversion into RGB. 

For the case of YUV to RGB conversion, the combination and conversion is as simple as a matrix multiplication.