Reverse gist_rainbow colors to match order of manual presets (#2731)

* Reverse gist_rainbow colors to match order of manual presets

* Add changelog

* Reverse in the range

Co-authored-by: P. L. Lim <2090236+pllim@users.noreply.github.com>

* Add comment

* Update test

---------

Co-authored-by: P. L. Lim <2090236+pllim@users.noreply.github.com>

CHANGES.rst

@@ -124,6 +124,8 @@ Bug Fixes
 
 - Fix redshifted line lists that were displaying at rest wavelengths, by assuming a global redshift. [#2726]
 
+- Order of RGB preset colors now matches for less than and greater than 5 layers. [#2731]
+
 Cubeviz
 ^^^^^^^
 

jdaviz/configs/default/plugins/plot_options/plot_options.py

@@ -806,8 +806,10 @@ def apply_RGB_presets(self):
         # Sample along a colormap if we have too many layers
         if n_visible > len(preset_colors):
             cmap = matplotlib.colormaps['gist_rainbow'].resampled(n_visible)
+            # Have to reverse the order of the cmap to make physical sense with
+            # assumed wavelength order of layers.
             preset_colors = [matplotlib.colors.to_hex(cmap(i), keep_alpha=True) for
-                             i in range(n_visible)]
+                             i in range(n_visible - 1, -1, -1)]
         elif n_visible >= 2 and n_visible < len(preset_colors):
             preset_colors = [preset_colors[i] for i in preset_inds[n_visible]]
 

jdaviz/configs/default/plugins/plot_options/tests/test_plot_options.py

@@ -288,8 +288,16 @@ def test_apply_presets(imviz_helper):
     po.apply_RGB_presets()
 
     assert po.layer.selected == "array_3"
+    assert po.stretch_function.value == "arcsinh"
+    po.layer = "array_5"
+    # Make sure this one didn't change
+    assert po.stretch_function.value == "linear"
+
+    # Turn layer 5 back on
+    po.image_visible = True
+    po.apply_RGB_presets()
 
-    colorbar_colors = matplotlib.colormaps['gist_rainbow'].resampled(7)
+    colorbar_colors = matplotlib.colormaps['gist_rainbow'].resampled(8)
     color_ind = 0
 
     def _rgb_to_hex(rgb):
@@ -298,15 +306,11 @@ def _rgb_to_hex(rgb):
 
     for i in range(8):
         po.layer = f"array_{i}"
-        if i == 5:
-            # Make sure this one didn't change
-            assert po.stretch_function.value == "linear"
-        else:
-            assert po.stretch_function.value == "arcsinh"
-            assert po.stretch_preset.value == 99
-            assert po.image_color.value == matplotlib.colors.to_hex(colorbar_colors(color_ind),
-                                                                    keep_alpha=True)
-            color_ind += 1
+        assert po.stretch_function.value == "arcsinh"
+        assert po.stretch_preset.value == 99
+        assert po.image_color.value == matplotlib.colors.to_hex(colorbar_colors(7-color_ind),
+                                                                keep_alpha=True)
+        color_ind += 1
 
 
 def test_track_mixed_states(imviz_helper):