You're likely seeing a blend with the background. When drawing alpha blended objects you *must* draw them back to front, except when using modulate or additive type blending.
You can disable ZWrites when drawing an object, allowing you to draw the front-most object first, then the more distant object, but the color mix will be incorrect, except in the case of add or modulate blending. The more layers of overlapping alpha objects, the worse the problem will be.
In general, draw all solid objects first. Next draw all alpha tested (on/off transparency, no blending) objects. Finally draw all alphablended objects sorted back to front, and likely with ZWrites disabled.
Technically you can draw the alpha tested objects whenever you want, but alphatest disables certain Z optimizations of modern GPUs and should be done after pure solid objects. While you can draw all these objects in any order, a rough front to back sort is best as a large nearby object will cause quick Z rejection of the further objects. These are all optimizations, and you can worry about them at a later time, but the Z sort of back to front for alpha blended objects isn't an optimization, it's the only way to have the math work properly.
As an example, lets clear to red, draw a green 50% square at depth 1, and a blue 50% square at depth 2. We'll do this 4 ways. With ZWrites on and off, and with green first then blue first.
Operation Framebuffer Z=On Framebuffer Z=OffClear 255, 000, 000 255, 000, 000Draw Green 128, 128, 000 128, 128, 000Draw Blue 128, 128, 000 064, 064, 128Clear 255, 000, 000 255, 000, 000Draw Blue 128, 000, 128 128, 000, 128Draw Green 064, 128, 064 064, 128, 064
The valid results are achieved when drawing blue then green, regardless of Z. The benefit of disabling Z writes is more for additive and modulate blending where depth ordering doesn't needed.
In the first case, with Z on and green first, we don't see the blue at all. It's hidden by the Z buffer holding the depth of the nearer green pixel first.
In the second case, with Z off and green first, we see a mix of all the colors, but it appears as though the blue is in front of the green, which isn't the case.
