Like you, my preferred tube paint is titanium white, which I use for drybrushing a basecoat over a black undercoat, mainly. I do use some other colours for terrain and similarly big jobs, but my main hesitation to use them for detail work is that it's less easy to control the amount used, which usually has to be mixed with some water as they are rather thick.
The actual paint opacity varies, it seems, with my favourite cadmium yellow and red having a great opacity, together with the white, with the others which I have tried trailing behind. This might obviously depend on the brand too.