Text and description quoted verbatim from Microsoft DirectX 7 help file, included here for convenient reference.

The D3DCOLORMODEL type is used to define the color model in which the system runs. A driver can expose either or both flags in the dcmColorModel member of the D3DDEVICEDESC7 structure.

typedef DWORD D3DCOLORMODEL

D3DCOLOR_MONO
Use a monochromatic model (or ramp model). In this model, the blue component of a vertex color is used to define the brightness of a lit vertex.
D3DCOLOR_RGB
Use a full RGB model.
Remarks
Prior to DirectX 5.0, these values were part of an enumerated type. The enumerated type in earlier versions of DirectX had this syntax:

typedef enum _D3DCOLORMODEL {
D3DCOLOR_MONO = 1,
D3DCOLOR_RGB = 2,
} D3DCOLORMODEL;