Hex color notation have an alpha channel

The hex color notation can be described as #RGB[A] or #RRGGBB[AA] - it accepts an alpha channel that can be used to represent the transparency.

When using it as #RRGGBB[AA], the alpha channel is a hexadecimal number where 00 is full transparent and FF full opaque. If using the shorter #RGB[A] notation, it is a hexadecimal number ranging from 0 and F.

#FF7f00     /* orange               */
#FF7f0000 /* orange 0% opaque */
#FF7f0080 /* orange 50% opaque */
#FF7f00FF /* orange 100% opaque */

#01E /* blue */
#01E0 /* blue 0% opaque */
#01E8 /* blue 53% opaque */
#01EF /* blue 100% opaque */

You can find out more on MDN.