#𝔼𝕊𝕄𝕚𝕟 2, 18 chars / 34 bytes ïē/\w⌿,↪(ḏ-`ᶍ⦃$}”ⓧ [Try it here (Firefox only).][1]

Using a version created after the challenge. #Explanation ïē/\w⌿,↪(ḏ-ᶍ⦃$}”ⓧ // implicit: ï=input, ḏ=15 ïē/\w⌿, // replace all alphanumeric chars in ï with: ↪(ḏ-ᶍ⦃$}”ⓧ // (15 - char's decimal form) converted to hex // implicit output #Non-competitive solution, 15 chars / 29 bytes ïē/\w⌿,↪(ḏ-`ᶍ⦃$}”ⓧ Uses transliteration. [1]: http://molarmanful.github.io/ESMin/interpreter2.html?eval=false&input=%23ffffff&code=%C3%AF%C4%93%2F\w%E2%8C%BF%2C%E2%86%AA%28%E1%B8%8F-%60%E1%B6%8D%E2%A6%83%24}%E2%80%9D%E2%93%A7

𝔼𝕊𝕄𝕚𝕟 2, 18 chars / 34 bytes

ïē/\w⌿,↪(ḏ-`ᶍ⦃$}”ⓧ

Try it here (Firefox only).

Using a version created after the challenge.

Explanation

ïē/\w⌿,↪(ḏ-`ᶍ⦃$}”ⓧ // implicit: ï=input, ḏ=15
ïē/\w⌿,             // replace all alphanumeric chars in ï with:
       ↪(ḏ-`ᶍ⦃$}”ⓧ // (15 - char's decimal form) converted to hex
                    // implicit output

Non-competitive solution, 15 chars / 29 bytes

ïē/\w⌿,↪(ḏ-`ᶍ⦃$}”ⓧ

Uses transliteration.

#𝔼𝕊𝕄𝕚𝕟 2, 15 chars / 29 bytes (non-competitive) ïĪ a=⩥Ḑⓜ$ⓧ)⨝,aᴙ [Try it here (Firefox only).][1]

Using a version created after the challenge. #Explanation ïĪ a=⩥Ḑⓜ$ⓧ)⨝,aᴙ // implicit: ï=input, Ḑ=16 ïĪ // transliterate: a=⩥Ḑⓜ$ⓧ)⨝ // the hex representation of the range [0,16)... ,aᴙ // with its reversed form // implicit output [1]: http://molarmanful.github.io/ESMin/interpreter2.html?eval=false&input=%23ffffff&code=%C3%AF%C4%AA%20a%3D%E2%A9%A5%E1%B8%90%E2%93%9C%24%E2%93%A7%29%E2%A8%9D%2Ca%E1%B4%99

#𝔼𝕊𝕄𝕚𝕟 2, 18 chars / 34 bytes ïē/\w⌿,↪(ḏ-`ᶍ⦃$}”ⓧ [Try it here (Firefox only).][1]

Using a version created after the challenge. #Explanation ïē/\w⌿,↪(ḏ-ᶍ⦃$}”ⓧ // implicit: ï=input, ḏ=15 ïē/\w⌿, // replace all alphanumeric chars in ï with: ↪(ḏ-ᶍ⦃$}”ⓧ // (15 - char's decimal form) converted to hex // implicit output #Non-competitive solution, 15 chars / 29 bytes ïē/\w⌿,↪(ḏ-`ᶍ⦃$}”ⓧ Uses transliteration. [1]: http://molarmanful.github.io/ESMin/interpreter2.html?eval=false&input=%23ffffff&code=%C3%AF%C4%93%2F\w%E2%8C%BF%2C%E2%86%AA%28%E1%B8%8F-%60%E1%B6%8D%E2%A6%83%24}%E2%80%9D%E2%93%A7