diff --git a/src/rimsschemedrawer/plotter.py b/src/rimsschemedrawer/plotter.py
index 8f96b2f..840efcc 100644
--- a/src/rimsschemedrawer/plotter.py
+++ b/src/rimsschemedrawer/plotter.py
@@ -56,22 +56,16 @@ def __init__(self, data: dict, **kwargs):
 
         self._figure, self._axes = kwargs.get("fig_ax", plt.subplots(1, 1))
 
-        # Colors for arrows
+        # Colors for headspace and main
         if darkmode:
             self.colmain = "#ffffff"
-            self.colir = "#d27878"
-            self.coluv = "#8c96df"
-            self.colfuv = "#9365b2"
-            self.colpump = "#60a55b"
             self.colhdr = "#4b5482"  # header color
         else:
             self.colmain = "#000000"
-            self.colir = "#a00000"
-            self.coluv = "#0012a0"
-            self.colfuv = "#5f00a0"
-            self.colpump = "#0aa000"
             self.colhdr = "#adbbff"  # header color
 
+        self.darkmode = darkmode
+
         # now plot the scheme
         self._plotit()
 
@@ -221,14 +215,7 @@ def _plotit(self):
 
         # draw the arrows for the steps
         for it in range(len(lambda_steps)):
-            if lambda_steps[it] >= 700:
-                col = self.colir
-            elif 500.0 < lambda_steps[it] < 700.0:
-                col = self.colpump
-            elif 350.0 < lambda_steps[it] <= 500.0:
-                col = self.coluv
-            else:
-                col = self.colfuv
+            col = ut.color_wavelength(lambda_steps[it], self.darkmode)
             # xvalue for arrow
             xval += deltax
             wstp = wavenumber_steps[it]
@@ -362,15 +349,7 @@ def _plotit(self):
             )
 
         for it in range(len(wavenumber_es)):
-            if lambda_step_es[it] >= 700:
-                col = self.colir
-            elif 500.0 < lambda_step_es[it] < 700.0:
-                col = self.colpump
-            elif 350.0 < lambda_step_es[it] <= 500.0:
-                col = self.coluv
-            else:
-                col = self.colfuv
-
+            col = ut.color_wavelength(lambda_step_es[it], self.darkmode)
             # values for spacing and distance
             xval = firstarrowxmfl + x_spacing_es + it * x_spacing_es
             yval = mfld_yinc * ipvalue * (1 + it)
diff --git a/src/rimsschemedrawer/utils.py b/src/rimsschemedrawer/utils.py
index dcabdf4..144ff40 100644
--- a/src/rimsschemedrawer/utils.py
+++ b/src/rimsschemedrawer/utils.py
@@ -149,6 +149,61 @@
 PLOT_STYLES = ["light", "dark", "light transparent", "dark transparent"]
 
 
+def color_wavelength(lmb: float, darkmode: bool = False) -> str:
+    """Color the wavelength according to the wavelength.
+
+    When darkmode is turned on, pastel colors are used. Definitions of colors are
+    taken from here:
+    https://sciencenotes.org/visible-light-spectrum-wavelengths-and-colors/
+
+    :param lmb: Wavelength in nm.
+
+    :return: Color string.
+    """
+    dict_light = {
+        "ir": "#500000",
+        "red": "#b70000",
+        "orange": "#d75700",
+        "yellow": "#c09e00",
+        "green": "#0aa000",
+        "light_blue": "#0098ff",
+        "deep_blue": "#0012a0",
+        "violet": "#5f00a0",
+    }
+    dict_dark = {
+        "ir": "#b98989",
+        "red": "#d27878",
+        "orange": "#ffa669",
+        "yellow": "#f4e67c",
+        "green": "#60a55b",
+        "light_blue": "#82b1d1",
+        "deep_blue": "#8c96df",
+        "violet": "#9e85af",
+    }
+
+    if darkmode:
+        dict_use = dict_dark
+    else:
+        dict_use = dict_light
+
+    if lmb > 700:  # infrared
+        return dict_use["ir"]
+    elif lmb >= 625:  # red
+        return dict_use["red"]
+    elif lmb >= 590:  # orange
+        return dict_use["orange"]
+    elif lmb >= 565:  # yellow
+        return dict_use["yellow"]
+    elif lmb >= 500:  # green
+        return dict_use["green"]
+    elif lmb >= 484:  # light blue
+        return dict_use["light_blue"]
+    elif lmb >= 450:  # deep blue
+        return dict_use["deep_blue"]
+    else:  # violet
+        return dict_use["violet"]
+
+
 def cm_2_to_nm(cm: Union[float, np.ndarray]) -> Union[float, np.ndarray]:
     """Convert a wavenumber in cm^-1 to a wavelength in nm.