added 132 characters in body

edited Jun 30, 2019 at 8:43

Wolfram Language (Mathematica), 6363 48 bytes

StringJoin@@{"#",IntegerString[Round[15{##}]&@@RGBColor@#,16]}&"#"<>Round[15List@@RGBColor@#]~IntegerString~16&

-15 bytes thanks to attinat! Replacing StringJoin with <> and compressing the syntax.

RGBColor@# converts the input string to a color of the form RGBColor[r, g, b] with three floating-point arguments in the range 0..1.
Round[15 {##}] &List @@ %%] multiplies thesethe list of three arguments by 15 and rounds them to the nearest integer. We now have a list of three integer values corresponding to the three desired hexadecimal digits.
IntegerString[%, 16]%~IntegerString~16 converts this list of three integers to a list of three hexadecimal strings of one character each.
StringJoin @@ {"#", %}"#"<>% prepends a # character and joins all these characters together.

Wolfram Language (Mathematica), 63 bytes

StringJoin@@{"#",IntegerString[Round[15{##}]&@@RGBColor@#,16]}&

RGBColor@# converts the input string to a color of the form RGBColor[r, g, b] with three floating-point arguments in the range 0..1.
Round[15 {##}] & @@ % multiplies these three arguments by 15 and rounds them to the nearest integer. We now have a list of three integer values corresponding to the three desired hexadecimal digits.
IntegerString[%, 16] converts this list of three integers to a list of three hexadecimal strings of one character each.
StringJoin @@ {"#", %} prepends a # character and joins all these characters together.

"#"<>Round[15List@@RGBColor@#]~IntegerString~16&

-15 bytes thanks to attinat! Replacing StringJoin with <> and compressing the syntax.

RGBColor@# converts the input string to a color of the form RGBColor[r, g, b] with three floating-point arguments in the range 0..1.
Round[15 List @@ %] multiplies the list of three arguments by 15 and rounds them to the nearest integer. We now have a list of three integer values corresponding to the three desired hexadecimal digits.
%~IntegerString~16 converts this list of three integers to a list of three hexadecimal strings of one character each.
"#"<>% prepends a # character and joins all these characters together.

added 573 characters in body

edited Jun 29, 2019 at 20:10

StringJoin@@{"#",IntegerString[Round[15{##}]&@@RGBColor@#,16]}&

RGBColor@# converts the input string to a color of the form RGBColor[r, g, b] with three floating-point arguments in the range 0..1.

Round[15 {##}] & @@ % multiplies these three arguments by 15 and rounds them to the nearest integer. We now have a list of three integer values corresponding to the three desired hexadecimal digits.

IntegerString[%, 16] converts this list of three integers to a list of three hexadecimal strings of one character each.

StringJoin @@ {"#", %} prepends a # character and joins all these characters together.

StringJoin@@{"#",IntegerString[Round[15{##}]&@@RGBColor@#,16]}&

StringJoin@@{"#",IntegerString[Round[15{##}]&@@RGBColor@#,16]}&

RGBColor@# converts the input string to a color of the form RGBColor[r, g, b] with three floating-point arguments in the range 0..1.

Round[15 {##}] & @@ % multiplies these three arguments by 15 and rounds them to the nearest integer. We now have a list of three integer values corresponding to the three desired hexadecimal digits.

IntegerString[%, 16] converts this list of three integers to a list of three hexadecimal strings of one character each.

StringJoin @@ {"#", %} prepends a # character and joins all these characters together.

answered Jun 29, 2019 at 20:04

StringJoin@@{"#",IntegerString[Round[15{##}]&@@RGBColor@#,16]}&