Add value or text to column a if condition in column b is met pandas

I keep getting nan for this, so hoping for a quick fix.

df:

cola    colb
5       30
10      95

I'd like to add 20 to cola if colb is between 90 and 100, like this:

cola    colb
5       30
30      95

Code i'm working with:

df['cola'] = df.loc[df['colb'].between(90,100), 'cola'] + 20

And bonus points - i have another column with text, i'd like to append a string if the condition is met, like this:

cola    colb    colc
5       30      some_text
30      95      some_text, condition_met

Code for this one that also returns nan is:

df['colc'] = df.loc[df['colb'].between(90,100), 'colc'] + 'condition_met'

Thanks