Hi guys,
i create a simple 4way vertex color material and paint with full constant vectors the textur on a simple plane. In the purple gradiant from red to blue it show the green color texture.
your lerps should be in series with the green channel overriding the red channel, and the blue channel overriding the green channel. then the secondary and tertiary colors should behave more like you would expect them to.
black = 1st texture
red = 2nd texture
yellow or green = 3rd texture
blue or white or purple or turquoise = 4th texture
so dark red would blend between 1 and 2.
orange would blend between 2 and 3.
blueish green would blend between 3 and 4.
dark blue would blend between 1 and 4.
dark green would blend between 1 and 3. etc…
but when you are using masks, grey will blend between everything (mostly the 4th texture), so try to use fully saturated colors where only 1 of your RGB channels is grey, while the other 2 channels are either black or white but not grey.
when you paint on vertex colors of a mesh, rather than pixels, you cannot control the linear blending between adjacent vertex colors, so they could blend towards browns and greys that mix all of your textures into a mess. to make this less noticable, you can try adding tighter loops around the vertex blended areas in the mesh or you can switch from vertex painting to using an image texture.