diff --git a/R/tinytheme.R b/R/tinytheme.R
index 556a8230..13426246 100644
--- a/R/tinytheme.R
+++ b/R/tinytheme.R
@@ -240,6 +240,14 @@ tinytheme = function(
     settings[[n]] = dots[[n]]
   }
 
+  # `col.default` < 0 gotcha for user-supplied palettes.(#627)
+  if ("palette.qualitative" %in% names(dots) &&
+      !("col.default" %in% names(dots)) &&
+      is.numeric(settings[["col.default"]]) &&
+      settings[["col.default"]] < 0L) {
+    settings[["col.default"]] = abs(settings[["col.default"]])
+  }
+
   # Compute mgp from spacing primitives when dynmar is active and the user
   # didn't provide an explicit mgp override. The near edge of margin text
   # (facing the plot region) aligns with the half-cell boundary (0.5*cex from
diff --git a/inst/tinytest/test-tinytheme.R b/inst/tinytest/test-tinytheme.R
index 7c6491e0..26d87bb7 100644
--- a/inst/tinytest/test-tinytheme.R
+++ b/inst/tinytest/test-tinytheme.R
@@ -238,3 +238,14 @@ f = function () {
     theme = list("clean", palette.qualitative = pal, pch = 21))
 }
 expect_snapshot_plot(f, label = "tinytheme_ephemeral_palette_function_qualitative")
+
+
+# User palette override should not be trimmed by a theme's negative
+# col.default (e.g. bw's -1), which would silently recycle colours. (#627)
+f = function() {
+  plt(
+    Sepal.Length ~ Petal.Length | Species, iris,
+    theme = list("bw", palette.qualitative = c("red", "blue", "green"))
+  )
+}
+expect_silent(f())