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Can you mix different kinds (or colors) of antifreeze?
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Yes, if you know exactly what you're mixing. Many antifreezes are not compatible. Some are compatible except for their corrosion inhibitor additives. Mixing incompatible antifreezes can destroy the extended ("Long Life") corrosion protection, whereupon the mix's corrosion protection defaults to the baseline two years.

Also, many Honda engines hide their water pump under the timing belt. The pump itself is only $60, but you've got a couple of hours labor to get at it. NEVER use silicated antifreezes in your Honda, since the silica corrosion inhibitor will grind up your pump's seals much more quickly than non-silicated stuff. The very best thing for your Honda is Genuine Honda premix.

If you want to use some non-silicated aftermarket antifreeze that's different from what you've already got in there, this PDF chart will help you find an appropriate mate:
http://www.eetcorp.com/antifreeze/Coolants_matrix.pdf

Another good link on the subject:
http://autorepair.about.com/cs/productreviews/l/aa052601a.htm

If you don't know what's in there, you're in a bit of a pickle. Add whatever you can get in order to get out of whatever jam you're in, but then drain and refill with a completely fresh mixture at the earliest opportunity. And don't neglect the block drain. There's a quart (or more) of fluid in your block, or close to 25% of the total capacity. 25% is quite a bit of old stuff to contaminate your new stuff with.

Here's more on the different kinds of antifreeze, another PDF. I don't remember how I got this one, or what Web site it came from:
DTurcotte_Mag_53_g.pdf