Purple, which is a mixture of red and blue, will appear to be red, since the blue becomes invisible. Older people would also have more difficult time seeing a purple object on a red background. When driving, we spend most of our time looking straight ahead but are still able to notice objects in the visual periphery. For example, an older driver would have a more difficult time detecting a car cutting across the intersection ahead, a car pulling out from a parking sport or a pedestrian walking across the road. It's unclear whether this effect is purely sensory or is due in part to a cognitive and attentional decline.
Older people are slower to respond when driving.
The best guess is that they are about 0.2-0.3 seconds slower than younger drivers. As might be expected, older drivers have a higher accident rate, but the number is still smaller than would be predicted from the visual decline. In general, older drivers are aware of their limitations and compensate by taking fewer risks. They tend to drive less, and to drive more slowly and less aggressively.