[arekkusu]

I think this setup does what you want; interpolate between two textures using the alpha channel of a third texture, and modulate the RGB color by the RGB of the same third texture. I do it with three units using combine and crossbar:

	// setup first texture unit as REPLACE (no vertex colors for us)
	glActiveTextureARB(GL_TEXTURE0);
	glEnable(GL_TEXTURE_2D);
	glBindTexture(GL_TEXTURE_2D, texture[GRASS]);
	glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);

	// setup second texture unit as INTERPOLATE (using the mixmap alpha via crossbar)
	glActiveTextureARB(GL_TEXTURE1);
	glEnable(GL_TEXTURE_2D);
	glBindTexture(GL_TEXTURE_2D, texture[WATER]);
	glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE);
	glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_RGB, GL_INTERPOLATE);
	glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_RGB, GL_TEXTURE);
	glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE1_RGB, GL_PREVIOUS);
	glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE2_RGB, GL_TEXTURE2);
	glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND2_RGB, GL_SRC_ALPHA);
	glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_ALPHA, GL_INTERPOLATE);
	glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_ALPHA, GL_TEXTURE);
	glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE1_ALPHA, GL_PREVIOUS);
	glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE2_ALPHA, GL_TEXTURE2);
	glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND2_ALPHA, GL_SRC_ALPHA);

	// setup third unit as MODULATE (colormap tinting, passthrough alpha)
	glActiveTextureARB(GL_TEXTURE2);
	glEnable(GL_TEXTURE_2D);
	glBindTexture(GL_TEXTURE_2D, texture[MIXMAP]);
	glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE);
	glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_RGB, GL_MODULATE);
	glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_RGB, GL_TEXTURE);
	glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE1_RGB, GL_PREVIOUS);
	glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_ALPHA, GL_REPLACE);
	glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_ALPHA, GL_PREVIOUS);

Note that I am using crossbar to specify the third texture as the interpolation parameter. Also note that this requires an image loader which will give you the raw RGB,A values of your mixmap without destroying them with premultiplication. QuickTime works, NSImage will not.