It's claimed that users know that the top of a stop sign is "red" or stop, and the bottom is go, but I wouldn't depend on this association; top-bad/bottom-good is an awkward association, red:bad::green:good works much better. There's lots of discussion on this issue, this site claims colorblind users still go by color at the stop light, but as they see "different" colors and the mapping of colors is so different this wouldn't apply in your case. I would avoid the metaphor altogether.
The multimodal approach is a great way to deal with colorblindness; if you change the color and the shape, position, ect of an element they're more clearly distinct. To keep with the traffic metaphor, maybe a stop sign (red octagon) could mean "bad," a yield sign (yellow triangle) could mean "okay," but traffic signs aren't always standard.
I would stick to more idiomatic symbols: Green plus/check for good, Red X/slashed out circle for bad. "Okay" is a trickier one. Maybe -/=/+? Space permitting, a label is often great. Don't forget alt text for vision impaired users too.
(Note: for the purpose of this example I assume you to mean "colorblind" as in red/green colorblind, which covers the large majority of colorblind users.)