It looks like a couple of these _ did not get changed to -, probably when you had to restore it when Glade forgot it.

Why the dash?

GdkRGBA supports colour names, so the hex-ification was not necessary:
In [1]: from gi.repository import Gdk                                                                                                       

In [2]: col = Gdk.RGBA()                                                                                                                    

In [3]: [(c, col.parse(c), col.to_string()) for c in 'red green blue yellow purple'.split()]                                                
Out[3]: 
[('red', True, 'rgb(255,0,0)'),
 ('green', True, 'rgb(0,128,0)'),
 ('blue', True, 'rgb(0,0,255)'),
 ('yellow', True, 'rgb(255,255,0)'),
 ('purple', True, 'rgb(128,0,128)')]

though green is slightly different.

Can these settings be changed again after this? I think attributes are merged with existing, so if the colour were removed, this would not reset it.
Alternatively, this could be written with Pango markup, but I'm not sure that would be any better.

No need for the if.

Ship It!