Blending Mask?

I think you''ve got it very wrong. The purpose of the alpha blending is to allow you to render a texture with transparent bits in it. I think that you could use multiple masks to achieve what you''re after, but the end result would be painful to produce. Think of it like a good old pen & paper stencil. You''d have to create alpha masks for every ''layer'' in your scene, and then flood-fill the entire scene with a texture, mask out the bits that you want to keep and render the next texture, add the mask for that layer to the previous mask and render the next texture... etc.

Pretty nasty. It''s not too clear from your post what you''re trying to achieve, but you may want to consider going for depth-sorted tiling or scenery chunks instead. This way you''d create the green grass with an alpha mask contained within it, and then just render the grass texture with alpha blending enabled.

Using color-keyed fills (which is what I think you''re wanting to do), isn''t really going to work purely by an alpha test. You may be able to get something going using pixel shaders, but... wow, that''s a serious investment in technology for a relatively simplistic effect.