Hex color code is a 6 symbol code made from three pairs. Each pair is a hexadecimal number representing a decimal value from 0 to 255. First pair represents amount of red color, second pair - green, third - blue. A color could be expressed in its RGB value or HEX value. Those values tell the display how much of a color to show. 0 means the deepest black and 255 the brightest white. All colors we see fit inside that range.
Hex color looks like this: #E83845. We put # in front of hex color codes. So there are three pairs: E8-38-45. Each pair could be converted to a decimal value. Converting from hexadecimal to decimal is quite confusing and better to use online converters instead. So E8 = 232, 38 = 56, 45 = 69.
So, #E83845 is the same as RGB(232, 56, 69) and will show the same color.
Usually in puzzle games, when dealing with HEX or RGB you will need to get the HEX or RGB code by inspecting item's color with a picker.