Using .append() with list vs dataframe

[Mark17]

My program adds rows to a dataframe by using .append() with a dictionary. In order to make this work, I think I had to do df = df.append() rather than just df.append(). Going back and retrying this afternoon, though, I think I've seen it work either way?

Here, though:

colors = []
for i in summary_results['ROI%']:
    if i < 0:
        colors = colors.append('r')
    else:
        colors = colors.append('g')
print(colors)

I get AttributeError: 'NoneType' object has no attribute 'append' . I need just colors.append('r') rather than assignment.

What's going on with when and why .append() may or may not require assignment (for lists vs. dataframes) in order to work?