It seems that only one color blend mode is available, which is close to brighten:

It may be nice to have other blend modes available like Multiply. In my case I want the orange particles to overlap and the result to still be the same orange (not sure how that Blend mode is called).

UPDATE: ah, I found the blendmode (additive, normal, subtractive) in the Particles > Basic menu, not in the Color tab. Normal blend mode does what I wanted.