The blunder appeared in an online blurb about the unique pattern, which was made specially for the Scottish independence campaign and unveiled at Yes Scotland's Burns Night supper earlier this week.
Libby Young, who works for the Bare Knees Kilts company and who created the design, wrote that grey had been picked because it "represents Scotland's future with business and industry".
A spokesman for Yes Scotland confirmed the design was genuine but had a different explanation for why grey was chosen – to represent Scottish weather.
"It's a great tartan. We hadn't seen what the design company were saying about it. Most people at the Yes Scotland Burns Supper on Wednesday evening thought the grey was meant to represent Scotland's weather," he said.
The tartan was revealed to around 70 attendees at the dinner.
Scottish Conservative MSP Alex Johnstone said: "It's clear that the grey reflects the vague uncertainty surrounding a separate Scotland's future, and the wind turbines that will dominate every hillside."