In which model does the color black form the key color?
the color black forms the key color in the
model.

RGB function basically used generated any color such as black or key color combination

Explanation:

In computer, all color code is generated based on RGB functions.

RGB function is color function code red, green and blue combination. It is eight bit color code is used.

If end-user  he or she uses RGB(255,0,0) where red color code is generated

If end-user he or she uses RGB(0,255,0) where green color code is generated  

If end-user he or she uses RGB(0,0,255) where green color code is generated

Based on these combination end user can generate code and displayed in the screen

End use can use RGB(255,255,255) for white and RGB(0,0,0) for black.