concat 3 columns of dataframe to one column

[flash77]

Hi,
I've got a dataframe with 100 rows and 3 columns (please see attachment).

How can I concate the 3 columns to one column?

Per example the 3 elements of the first row (255 255 255) should be 1 element (255255255) and so on.

import numpy as np
import pandas as pd
from PIL import Image

colorImage = Image.open("p2.bmp")
colorPixels = colorImage.convert('RGB')
# Make into array
colorArray = np.array(colorPixels.getdata())
df = pd.DataFrame(colorArray)
print(df)
df2 = pd.concat(df["0"], df["1"], df["2"])
print(df2)

Thanks a lot for your help!!