I believe your custom texture isn't fully opaque. Could you upload it?
On a side note.
Lua Code:
b.texture:SetTexture('Interface\\AddOns\\tMinimap\\Texture')
b.texture:SetColorTexture(1, 0, 0)
This doesn't do what you think it does. SetColorTexture is the same as SetTexture, but accepts RGBA instead of texture IDs or paths, it means that the only widget that actually uses your custom texture is the statusbar.
Try doing this:
Lua Code:
b.texture:SetTexture('Interface\\AddOns\\tMinimap\\Texture')
b.texture:SetVertexColor(1, 0, 0)