The matter seems to revolve more around the state of the object being described (particularly, whether it continues to exist), rather than the action being described.
The action, however, may affect the state. So, "I have eaten a red apple" (or, more naturally in most situations, simply "I ate a red apple") results in the apple's being eaten, at which point it ceases to exist. Therefore "that was red" (i.e. before I finished eating it) makes sense, but "that is red" does not, because the apple isn't anything any more.
As for the expression "I have found someone who has a car for us to drive", the assumption is that finding the someone did not cause the car to cease to exist. Therefore, (modifying the sentence slightly to make the effect clearly w/r/t the question),
"I have found someone who has a car that is red for us to drive"
makes intuitive sense, while
"I have found someone who has a car that was red for us to drive" is problematic. Technically, it is more correct, because the speaker can only vouch for the car's condition as it was described or observed at the time of the finding, which is in the past. It could have been painted a different color, or even destroyed, in the interval between the moment the someone was found, and the moment the speaker makes this statement.
However, for practical purposes, this is seen to be quibbling. For one thing, if the car has been destroyed, it isn't available to be driven. So, the underlying assumption for the whole exercise to make sense must be that the car still exists.
So, "...that is red..." is used in practice.