_RGB32 can take an extra fourth alpha parameter too. _RGBA32 remains for retrocompatibility, but it became unnecessary with v1.3 I think.
Also:
1 parameter: _RGB32(255) is the same as _RGB32(255, 255, 255)
2 parameters: _RGB32(255, 30) is the same as _RGB32(255, 255, 255, 30)
3 parameters is the default RGB setting.
4 parameters is RGBA.