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.

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