Trouble with Indigo oil paint

Having seen the beautiful dark indigo background of a Chinese book printed during the Ming Dynasty [ at the current British Museum Exhibition ]

http://www.britishmuseum.org/whats_on/exhibitions/ming.aspx?utm_expid=1760025-5.5M5ZQHKEQtSoqoh2xqqYEA.0

I decided to experiment with indigo myself.

After two glazes of Indigo I realized that this blue has a bitterness that will set my teeth on edge when warmer colours are placed anywhere nearby. Fortunately it will take umpteen more layers until I reach the depth of colour on the label, and so I decide to see what will happen if I glaze it over with ultramarine blue.
 Despite searching all over the Indigo tube of paint [ Artists Quality oil paint ] I can find none of the usual marking code to indicate whether this is a transparent, staining, semi-transparent or opaque colour. I have to guess therefore that it is opaque; I know Ultramarine in artists quality oil paint is transparent and wonder whether its warmth will actually have any effect on the cold layer below.

I thought maybe three glaze layers might help, but it took five glazes before I really had some degree of warmth and real depth of colour. Now in fact I enjoy the beautiful velvety deep blue background that has been created. I wanted a really deep background to offset the paler tones, and a background that would recede. I think I have discovered a way to take off the Indigo bitter edge, but only time will tell as I continue with the painting.