Acrylic is fine, although you can also use enamel if you attach the track soon
after the paint dries. Before enamel hardens all the way through, the paint is
slightly flexible and won't flake too badly. Note that not all flexible track
is vinyl. Beware of using enamels or turpentine washes on the so-called
glueable styrene recently adopted by Tamiya and some others, as the mineral
spirits or turpentine can dissolve this soft plastic. With that type of track,
stick to water based acrylics.