Grasshoppers often spit out juice like that to make them less appealing to whatever wants to eat them. I never heard of katydids doing that but it is probably the same thing for the same reason. The yellow stuff could be eggs. I imagine it was flying and was struck by the car which caused it to expel some eggs and then it spit out the juice because its instinct was to do that when molested.

I found this on my car earlier .. It's about two inches long .. What happened ?? It has that black stuff coming out of his mouth and that yellow stuff , I assume , came out of his body too ..