A nice image of exaggerated maria colors on the Moon: http://www.rc-astro.com/img/moon_colors_2005-04-18_web.jpg.
Visually, you can see this, especially between Mare Serenitatis and Mare Tranquilatis (wow I can't believe I spelled that right the first time). They are the two prominent, quasi-circular touching maria on the left side. The color difference for maria basalts is the difference in composition of TiO2.
Tranquilatis, the bluer one, is very rich in TiO2.
Giguere, Thomas A.; Taylor, G. Jeffrey; Hawke, B. Ray; Lucey, Paul G.
The titanium contents of lunar mare basalts
Meteoritics & Planetary Science, vol. 35, no. 1, pp. 193-200 (2000).