wrong background colors of linked alignment

[Justine Pollet]

  Hi,

I don't understand why my nucleotide linked alignement appears in yellow , orange , grey and light grey instead of the default  colors in faces.py   https://github.com/etetoolkit/ete/blob/master/ete3/treeview/faces.py#L140 .

Thanks in advance

Justine

[Justine Pollet]

I can see the right colors if i do : 

```

faces._ntbgcolors = {

    'A':'#A0A0FF',

    'G':'#FF7070',

    'I':'#80FFFF',

    'C':'#FF8C4B',

    'T':'#A0FFA0',

    'U':'#FF8080',

    '.':"#FFFFFF",

    '-':"#FFFFFF",

    ' ':"#FFFFFF",

    'N':"#FFFFFF"

}

igphyml_tree = PhyloTree(tree_path,

                         alignment="seq.fasta", 

                         alg_format="fasta" )

for leaf in igphyml_tree.traverse():

        leaf.add_face(SequenceFace(seq=leaf.sequence, bg_colors=faces._ntbgcolors),column=1 )

```

But the sequence is not aligned like with linked alignement and the both link alignement and manually add SequenceFace is in plot.

If i not adding manually SequenceFace whenever I had N keys like above or directly in  ete3/treeview/faces.py script the linked alignement output with wrong colors.

How can i have right background colors on my linked alignement ?

Thanks