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.
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.
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.
Final answer is:
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.