gui: Also use colour for absolute frequency

This was based on a miscommunication between me and Tom
a long while ago, where I mentioned the detuning text
would be more readable on a screen across the lab if
it was white.

wand/gui.py

@@ -402,7 +402,7 @@ def update_freq(self):
             detuning = "Error"
             colour = "ff9900"
 
-        self.frequency.setText(freq)
+        self.frequency.setText(freq, color=colour)
         self.detuning.setText(detuning, color=colour)
 
         self.wake_loop.set()  # recalculate when next measurement due