Soil mix can have some effects on germination but it is not the primary issue. Given consistently correct temps and amount of humidity healthy seeds that have been properly harvested and stored will germinate on coffee filters, tissue paper, rocks or just about anything. The soil mix itself only comes into play AFTER germination. ProMix is excellent but so are many other mixes.

So with low germination rates the most likely culprit, especially with non-commercial (home) saved seeds is the quality of the seeds themselves. If these are "unique colors" they may be sterile hybrids. Or they may have been harvested before maturity or stored where they got too warm, etc.

Pelargonium are generally considered fairly easy to germinate although they can be slow so if you are having consistently low germination rates I'd question the quality of the seeds themselves. I'd suggest you try using the "baggie method" discussed in the FAQs here with some and see if that doesn't improve your rates. If it doesn't then the issue is likely with your seeds.

Check out:

http://www.flower-gardening-made-easy.com/geraniums-from-seed.html

Dave