Plotting multiple lines with same color, but using hue to separate the lines

When hue is set, seaborn wants to draw each line in a different color. These colors are chosen from the palette (in this case the default palette has 10 different colors).

You can create a custom palette, e.g. from a list of colors. Even a list with just one element. So, palette = sns.color_palette(['black']).

An annoyance is that seaborn insists that the palette has exactly the same number of colors as there are hue-values. Therefore, that number needs to be provided as a parameter (palette = sns.color_palette(['black'], number_of_colors)).

Here is some example code (the default legend has been turned off, as all lines now have the same color):

import seaborn as sns
import pandas as pd
from io import StringIO

data_str = '''Age    "Predicted Probability"  Obs
18.0   0.206                  3.0
15.0   0.206                  3.0
34.0   0.154                  3.0
52.0   0.124                  3.0
46.0   0.146                  3.0
18.0   0.000                  2.0
15.0   0.000                  2.0
34.0   0.000                  2.0
52.0   0.098                  2.0
46.0   0.002                  2.0
18.0   0.000                  1.0
15.0   0.000                  1.0
34.0   0.004                  1.0
52.0   0.078                  1.0
46.0   0.006                  1.0
18.0   0.038                  0.0
15.0   0.032                  0.0
34.0   0.384                  0.0
52.0   0.318                  0.0
46.0   0.348                  0.0
'''
df = pd.read_csv(StringIO(data_str), delim_whitespace=True)

palette = sns.color_palette(['black'], len(df['Obs'].unique()))
sns.lineplot(data=df, x="Age", y="Predicted Probability", hue='Obs', palette=palette, legend=False)

PS: An alternative that doesn't need counting nor manually separating the hues, is creating a ListedColormap with one color:

import seaborn as sns
from matplotlib.colors import ListedColormap

sns.lineplot(data=df, x="Age", y="Predicted Probability", hue='Obs', palette=ListedColormap(['black']), legend=False)

you can change the appearance of lines by hue or dashes

1. hue colors depend on color_palette

you can use seaborn.set_palette before drawing to set custom or any palette predefined that can be used when you draw different data on same plot . can see more examples and info from seaborn doc you may find preferred colors in documentation or can define your palette
seaborn.color_palette object used to pass to set_palette.

# Create an array with the colors you want to use
colors = ["#FF0B04", "#4374B3"]# Set your custom color palette

sns.set_palette(sns.color_palette(colors))
#plot anything with hue ...first color takes category and so on 
sns.lineplot(data=df, x="Age", y="Predicted Probability", hue = 'Obs')       

you can this helpful also How to use your own color palettes with Seaborn by Carolina Bento Aug 22, 2019·3 min read

2. you can use dashes also

Also you can try this dashes property inside lineplot function if you don't like using hue :

dashes: boolean, list, or dictionary

Object determining how to draw the lines for different levels of the style variable. Setting to True will use default dash codes, or you can pass a list of dash codes or a dictionary mapping levels of the style variable to dash codes. Setting to False will use solid lines for all subsets. Dashes are specified as in matplotlib: a tuple of (segment, gap) lengths, or an empty string to draw a solid line.

sns.lineplot(x="Age", y="Predicted Probability",style='Obs',dashes=[(1,1),(5, 10)],data=df)

this article can help you revise most of lineplot properties