Colors are combinations of red, green, and blue (RGB). Computers use hexadecimal numbers to represent color. A value of 00 means no color while a value of FF mean full color. 

White is a mixture of red, green, and blue colors, represented by #FFFFFF.

Red is a mixture of red only and is represented by #FF0000.

Blue is a mixture of blue only and is represented by #0000FF. 

Yellow is a mixture of red and green, represented by #FFFF00. By rule, there is always a # sign. The first pair FF indicates full red. The second pair FF means full green. Last pair 00 means no blue. Reading the chart below, yellow is the second column (FFFF--), last row (00).