The meaning of the word dull is either:

• lacking interest or excitement
• lacking brightness, vividness, or sheen.

So, let us go with the second meaning.  As you know, street photography is meant to have candid photos of people.  This can't always happen. If this is where you find tourself today, use who you have around; but, do include a person as this gives the photo an interest. 

Converting it to black and white for today's prompt word will help, to not make it bright but dull looking!