> custom_colors <- colorRampPalette(c("blue", "green", "yellow", "red"))(100)
> # Use custom color ramp
> mfuzz.plot2(mfuzz_class,
+ cl = mfuzz_cluster,
+ mfrow = c(2, 5),
+ colo = custom_colors,
+ time.labels = colnames(TPM_JA),
+ x11 = FALSE,
+ center = TRUE)
Error in if (colo == "fancy") {: the condition has length > 1

The help page for mfuzz.plot2 says

colo       color palette to be used for plotting. If the color argument remains empty, the
           default palette is used. If the colo = "fancy", an alternative (fancier) palette
           will be used.

The colo argument for mfuzz.plot will accept a color palette however.