For streamlines they used to use colored oil and matrices of yarn tufts on the track, which were videotaped. In the tunnel they use smoke wands, and sometimes even tools like laser velocimetry for very detailed studies. The tunnel models also have lots of little holes in their surface which are connected to very sensitive pressure transducers, from which the dynamic surface pressures can be measured.
But as for actual flow visualization, I don't know of any paint that will give you that feedback. There are types of pressure sensitive coatings that can be applied, but they require careful calibration and special cameras to get results. And they will only give you surface pressure distribution, not flow data.
CFD softwares are now widely available, fairly easy to use, and can give very good results when used properly. Most aero work is now performed digitally, with tunnel work only being used to validate the analytical modelling.
Here's a source for pressure sensitive paints and films: