Find the closest HTML colour name
C# (Visual C# Interactive Compiler), 231+21=252 bytes



s=&gt;Color.FromArgb(Convert.ToInt32(s,16));f=s=&gt;Enum.GetNames(typeof(KnownColor)).Select(x=&gt;Color.FromName(x)).Where(x=&gt;!x.IsSystemColor&amp;x.A&gt;254).OrderBy(x=&gt;Math.Abs(x.R-C(s).R)+Math.Abs(x.G-C(s).G)+Math.Abs(x.B-C(s).B)).First().Name


Explanation:

using System.Drawing; //System library that handles Colors in C#

C=s=&gt;Color.FromArgb(Convert.ToInt32(s,16));        //helper method. receives a hex string and returns it's Color

f=s=&gt;Enum.GetNames(typeof(KnownColor))             //collection of color names from the System.Drawing.KnownColor enum
         .Select(x=&gt;Color.FromName(x))             //convert the color names to Colors
             .Where(x=&gt;!x.IsSystemColor&amp;x.A&gt;254)   //filter out unwanted colors
                 .OrderBy(x=&gt;Math.Abs(x.R-C(s).R)  //order by increasing distance
                            +Math.Abs(x.G-C(s).G)
                            +Math.Abs(x.B-C(s).B)) 
                     .First().Name                 //return the closest color's name


For some reason, Tio complains that the namespace 'Drawing' does not exist in the namespace 'System', despite the source project Mono stating it's compatible. It works fine in VisualStudio though.

EDIT: apparently it hasn't been implemented into Tio yet!

Try it online!