SetPixels edge pixels

Hello. I am using SetPixels to draw on a Texture2D. I noticed, that when I try to draw for example a red rectangle 20x20, the border pixels differ from the ones that are inside - they are not 100% red, their color tries to merge with the color near it. And it seems ok, but there is an issue: for example, I have one fully red texture, and another one fully transparent above it. When I try to apply SetPixels on the transperent one, here is what I get: The border red pixels of the transparent texture merge with transparent ones, that causes such effect. My question: is there a way to avoid this merging? Thanks!