The phrase "mere metaphor" refers to a figure of speech in which one thing is used to represent or symbolize another thing. It implies that the metaphor is only a comparison and shouldn't be taken literally. Full definition
In such cases, however, the original mythological meaning has been lost, and they have become mere metaphors or ciphers. (religion-online.org)
The world might seem a little grayer than usual when we're down in the dumps and we often talk about «feeling blue» — new research suggests that the associations we make between emotion and color go beyond mere metaphor. (sciencedaily.com)