What B-V colour should we expect for the earthshine?
We will here estimate it by using the change in colour of Sunlight that has struck the Moon once, and the colour of Earth as estimated from spacecraft.
The Sun's B-V is +0.650 [Allen, 1973] [Holmberg et al, MNRAS, 367, 449, 2006]
The Full Moon's B-V is +0.85 [Lane&Irvine, AJ 1976 78, p. 267]
[vdBergh has +0.876 for Mare Serenitatis;
Allen 4.ed. table 12.16 has 'Moon' B-V 0.92;
Gallouet (1963) has +0.94;
Wildey & Pohn (1964), AJ vol 69, p.619 have a range of values near +0.86 to +0.87 (their work seems good and a milestone).]
The Earth's B-V is 0.2 [Allen 3. ed, but appears based on a 1961 work - so pre-spaceage?]
The Moon's DS B-V is 0.64 on average given data in [Franklin (1967), JGR 72, p 2963]
If Sunlight is reddened by one reflection off the Moon by 0.85-0.65=+.2 mags, then we expect earthshine, bounced once off the Moon to redden by the same amount.
If the Earth has B-V=0.2 as seen from space then seen after one reflection it ought to be redder by +0.2 or appear to us observing it from Earth at B-V=0.4. This is not what Franklin measured.
Basically, we do not yet know Earth's B-V colour! I am making inquiries, and we shall see.
Note that Danjon did lots of colour observations of earthshine - but in the Rougier system. Wildey [JGR vol 69, p.4661+] refers to a transformation from Colour Index ("C.I.") in the Rougier system to B-V in the Johnson system - but without giving numerical details. The method is based on transformations using the Full Moon and the Sun colours.
The transformation should be made specific and the data from Danjon placed online. Another student project!