When litmus paper touches and acid, the paper changes to red. When it touches a base, the paper changes to blue. By comparing the color the litmus paper changed to by a chart, you could determine how strong or weak that acid or base is.
Because it canges color if it is a base or a acid, I think its blue for base and red for acid Blue at pH > 8 and red at pH < 5.5 ; In between the color change is not very indicative.