@@ -8,3 +8,52 @@ The `config` folder contains configuration files which customize default **PyAut
 - `mat_wrap.yaml`: Specify the default matplotlib settings when figures and subplots are plotted.
 - `mat_wrap_1d.yaml`: Specify the default matplotlib settings when 1D figures and subplots are plotted.
 - `mat_wrap_2d.yaml`: Specify the default matplotlib settings when 2D figures and subplots are plotted.
+
+# Changing the colormap
+
+Every 2D figure — imaging data, fits, residual maps, inversion reconstructions —
+draws with the colormap named by the `colormap` key of `general.yaml`:
+
+```yaml
+colormap: autoarray   # any matplotlib colormap name, e.g. magma, viridis, inferno
+```
+
+`autoarray` is the colormap bundled with **PyAutoArray**; any other value is
+looked up in matplotlib, so `magma`, `viridis`, `inferno`, `plasma`, `jet` and
+the rest of `list(matplotlib.colormaps)` all work. Editing this one key is
+enough — no plotting code needs changing.
+
+A name matplotlib does not recognise (a typo, say) raises a `ValueError` naming
+the key and the offending value. It is **not** silently swapped back for the
+default, so a colormap setting never goes quietly ignored.
+
+## One figure at a time
+
+To override the colormap for a single figure without touching config, pass
+`colormap=` to any plot function:
+
+```python
+import autoarray.plot as aplt
+
+aplt.plot_array(array=image, colormap="magma")
+aplt.plot_inversion_reconstruction(pixel_values=values, mapper=mapper, colormap="viridis")
+```
+
+The same argument exists on the **PyAutoGalaxy** and **PyAutoLens** plot
+functions (`subplot_fit`, `plot_tracer`, `subplot_sensitivity`, …), which pass
+it straight through to **PyAutoArray**. Its "use the config value" default is
+spelled `None` in **PyAutoArray** and **PyAutoLens**, and `"default"` in
+**PyAutoGalaxy**; both mean the same thing.
+
+## Figures that deliberately ignore the setting
+
+A few figures fix their colormap because the colormap carries meaning that a
+user preference should not override:
+
+- The `array_overlay` of `plot_array` uses `Greys`, so the overlaid array stays
+  legible on top of whatever colormap the main array is drawn in.
+- The weak-lensing figures in **PyAutoLens** use `twilight` for position angles
+  (cyclic data needs a cyclic colormap) and `RdBu_r` for residuals (diverging
+  data needs a diverging colormap centred on zero).
+- The cluster figures in **PyAutoLens** use `gnuplot2`, and the interactive GUI
+  tools use `jet`, to keep faint features visible while masks are drawn by hand.
@@ -901,34 +901,133 @@ def hide_unused_axes(axes) -> None:
             ax.axis("off")
 
 
+#: Config key holding the default colormap, quoted in error messages.
+_COLORMAP_CONF_KEY = "visualize/general.yaml -> colormap"
+
+
 def _default_colormap() -> str:
-    """Return the colormap name from config, registering the custom one if needed."""
+    """Return the default colormap name for 2D figures.
+
+    The name is read from the ``colormap`` key of ``visualize/general.yaml``.
+    The two failure modes are deliberately kept apart:
+
+    - **No config at all** (``autonerves`` not installed, or no ``colormap``
+      key on the config path — e.g. a bare install with no workspace) falls
+      back quietly to the bundled ``"autoarray"`` colormap.  Nothing is
+      misconfigured, so nothing is said.
+    - **A config value that matplotlib does not recognise** raises
+      ``ValueError``.  A typo'd colormap name used to revert silently to
+      ``"autoarray"``, so the user never learned their setting was ignored.
+
+    Returns
+    -------
+    str
+        A colormap name matplotlib can resolve — either ``"autoarray"`` (the
+        bundled colormap, registered here on first use) or a matplotlib name.
+
+    Raises
+    ------
+    ValueError
+        If the ``colormap`` config key is set to something that is not a
+        registered matplotlib colormap.
+    """
     try:
         from autonerves import conf
-        name = conf.instance["visualize"]["general"]["colormap"]
-    except Exception:
+        from autonerves.exc import ConfigException
+    except ImportError:
         name = "autoarray"
+    else:
+        try:
+            name = conf.instance["visualize"]["general"]["colormap"]
+        except (KeyError, ConfigException):
+            name = "autoarray"
+
     if name == "autoarray":
         from autoarray.plot.segmentdata import register
+
         register()
+        return name
+
+    _validate_colormap(name)
+
     return name
 
 
+def _validate_colormap(name) -> None:
+    """Raise ``ValueError`` unless *name* is a colormap matplotlib knows about.
+
+    Parameters
+    ----------
+    name
+        The colormap name read from config (or passed by the user).
+
+    Raises
+    ------
+    ValueError
+        If *name* is not a string, or is not a registered matplotlib colormap.
+    """
+    import matplotlib
+
+    if isinstance(name, str) and name in matplotlib.colormaps:
+        return
+
+    raise ValueError(
+        f"Unknown colormap {name!r}.\n\n"
+        f"The config key `{_COLORMAP_CONF_KEY}` is set to {name!r}, which is "
+        f"not a colormap matplotlib recognises, so no figure can be drawn "
+        f"with it.\n\n"
+        f"Use either `autoarray` (the colormap bundled with PyAutoArray) or "
+        f"any matplotlib colormap name, for example `magma`, `viridis`, "
+        f"`inferno`, `plasma` or `jet`.\n"
+        f"The full list is `list(matplotlib.colormaps)`."
+    )
+
+
 def _conf_imshow_origin() -> str:
-    """Return the imshow origin from config (``"upper"`` or ``"lower"``)."""
+    """Return the imshow origin from config (``"upper"`` or ``"lower"``).
+
+    An absent config falls back quietly to ``"upper"``; a value matplotlib's
+    ``imshow`` would reject raises ``ValueError`` rather than being silently
+    swapped for the default (same contract as :func:`_default_colormap`).
+    """
     try:
         from autonerves import conf
-        return conf.instance["visualize"]["general"]["general"]["imshow_origin"]
-    except Exception:
+        from autonerves.exc import ConfigException
+    except ImportError:
+        return "upper"
+
+    try:
+        origin = conf.instance["visualize"]["general"]["general"]["imshow_origin"]
+    except (KeyError, ConfigException):
         return "upper"
 
+    if origin not in ("upper", "lower"):
+        raise ValueError(
+            f"Invalid imshow origin {origin!r}.\n\n"
+            f"The config key `visualize/general.yaml -> general -> "
+            f"imshow_origin` must be either `upper` or `lower`."
+        )
+
+    return origin
+
 
 def _conf_output_format() -> str:
-    """Return the default output_format from config (``"show"``, ``"png"``, etc.)."""
+    """Return the default output_format from config (``"show"``, ``"png"``, etc.).
+
+    An absent config falls back quietly to ``"show"``.  The value itself is not
+    validated here — an unsupported format surfaces as matplotlib's own
+    ``savefig`` error, which already names the offending format and lists the
+    supported ones.
+    """
     try:
         from autonerves import conf
+        from autonerves.exc import ConfigException
+    except ImportError:
+        return "show"
+
+    try:
         return conf.instance["visualize"]["general"]["general"]["output_format"]
-    except Exception:
+    except (KeyError, ConfigException):
         return "show"