Read this explanation and only after take the exam
Colors can have HEX value, be RGB, RGBA, HSL, HSLA or choosen by name.
RGB format has a combination from 0 to 255 of the main colors Red, Green, Blue so RedGreenBlue=RGB and RGB(0,255,255) is cyan.
RGBA have the alpha coefficient that goes from full trasparent:0 to full color:1 so RGB(255,0,0,0.5) is red half trasparent.
HEX is a combination of couples of exadecimal numbers (from 0 to F) for red and green and blue so black is HEX(FF,FF,FF).
HSL has hue(color goes from 0 that is red,120 that is green and 240 that is blue to 360 that becomes again red), saturation goes from 0 "no color" to 100% full color, and lightness is 0%black and 100%white so you can have hsl(120,100%,50%) that is normal green.
HSLA is same of HSL with the add of alpha factor that goes from full transparent to full opaque.
Colors can be also set by name like background-color:Olive or color:Tomato.