1. How about painting the wall a new color?

2. How about we paint the wall a new color?

(Are both grammatical?)

2 answers

Both are slang, somewhat, but the first one is more commonly used.
'How about painting the wall a new color?' is addressing one person asking a question to a separate individual. 'How about we paint the wall a new color? is referring to the person asking the question who would like to help the individual with the painting.