An entry in Fortnightly Topic Challenge #42: Wordless Connecting Walls

TASK: Partition these 16 colours into 4 groups of 4 based on shared properties. Then identify the only colour (not represented here) which satisfies all 4 categories.

• And yes, you know what you need to do first of all... (at least you only need to do it 16 times...!)
– Stiv
Commented Nov 1, 2020 at 8:45
• (oh, it has started already :0 )
– oAlt
Commented Nov 1, 2020 at 8:52
• to save people effort, the decimal codes are '210 105 30', '148 0 211', '46 139 87', '100 149 237', '218 165 32', '255 0 255', '154 205 50', '230 230 250', '0 255 127', '255 99 71', '0 206 209', '216 191 216', '173 255 47', '255 160 122', '0 139 139', '139 0 0', so the hexadecimal codes are D2-69-1E, 94-0-D3, 2E-8B-57, 64-95-ED, DA-A5-20, FF-0-FF, 9A-CD-32, E6-E6-FA, 0-FF-7F, FF-63-47, 0-CE-D1, D8-BF-D8, AD-FF-2F, FF-A0-7A, 0-8B-8B, 8B-0-0 Note, I have no idea whether either of those is relevant, but I'm guessing one might be, and figured might as well save people 'manual' labor. Commented Nov 1, 2020 at 9:05

First of all you have the colours from left to right, top to bottom:

Chocolate, Dark violet, Sea green, Cornflower blue;
Goldenrod, Fuchsia, Yellow green, Lavender;
Spring green, Tomato, Dark turquoise, Thistle;
Green yellow, Light salmon, Dark cyan and Dark red.

Groups are:

Dark: Dark violet, Dark turquoise, Dark cyan, Dark red.

Green: Sea green, Green yellow, Spring green, Yellow green.

Food: Chocolate, Light salmon, Tomato, Cornflower.

Plants: Thistle, Lavender, Goldenrod, Fuchsia.

DARK OLIVE GREEN #556b2f

Title:

16 is a reference to Hexadecimals! OP edit for completeness: And the initials of the words in the puzzle title spell CSS, which also stands for Cascading Style Sheets, a style sheet language used to control the presentation of web pages and which supports the use of all the colour names in this puzzle.

• You're very close...! 'Magenta' goes by another name which is probably more obviously appropriate for this category (which does not need to be as specific as 'FLOWERS'). And your final answer (despite its seasonal relevance) does not match the patterns exhibited by all of the other groups. (Use the title!)
– Stiv
Commented Nov 1, 2020 at 9:45