Yes Joy, if they have a distinctive color/font and people notice different sayings, they will start looking for them.
I’m thinking black on yellow. The yellow blackground can be spotted from far away, is something that grabs your attention and black/yellow is the “caution/danger” combination.
As for font, better to stick with something easy to read like helvetica.
The GOE's bumpersticker is Black on yellow, and the message is, "If you can't stand behind them, stand in front of them."