I did exactly what you're inquiring about. Alternated black & silver for my mountain tandem front wheel:


If you mix spoke colors, just be sure to match gauge. I even went the extra step and alternated alloy black & silver nipples. (What is this, some Raiders-themed bike???)

I did it because I already had half the spokes I needed in one color. So 16 more of the other was all I needed.

The 36 butted spoke 3X wheel with alloy nipples has performed fine on the mountain tandem for 15 years.