Question How to add 2 semi-transparent colors?

I have to copy pixels from one texture on top of another texture but some of the pixels are half transparent which means that instead of overwriting the pixel info, I need to add it to the current color.

I tried doing color + color but that leads to something completely broken.



Here is what the 2 textures look like when overlayed in the editor.



I have tried other methods but those didn't work either. Does anybody know how to do this through code?

EDIT: The background is a solid color

 

I figured it out. The code for multiplying 2 pixels is as follows

Code (CSharp):

1. Color currPixel = image1.texture.GetPixel(x1, y1);
2. Color newPixel = image2.texture.GetPixel(x2, y2);
3.  
4. Color mixedPixel = new Color(
5.     (currPixel.r * (1f - newPixel.a) + newPixel.r * newPixel.a),
6.     (currPixel.g * (1f - newPixel.a) + newPixel.g * newPixel.a),
7.     (currPixel.b * (1f - newPixel.a) + newPixel.b * newPixel.a),
8.     Mathf.Clamp01(currPixel.a + newPixel.a)
9.     );
10.  

I actually made a mistake while grabing the current pixel and used x2 and y2 instead which is why I couldn't get it to work.