Generally, the more contrast you can create between text and background the better the readability. The use of red and green together has its applications, but absolutely vibrate when touching. So like two quarreling siblings, separate the two when necessary.
Gray and red seem to be very different in color, but since they are so close in value, impact is lost when they touch.
Even popular drop shadows can create readability issues when the edge between text and background becomes blurred.