If you have no plants you have many options.
As the waters of Lake Malawi get quite deep, and the fish swim correspondingly deep (some down to several hundred meters), most of the colors are gone from the water by the time they hit the fish (all color gone by 100 meters or so). All of the pix I've seen of Lake Malawi look very blue, as you might expect. So, since it's such a large and deep lake, as with
SW fish, I'd use an actinic bulb to synthesize the blue disparity in the water.
I don't have a LM setup *yet* but am putting one together for my professor. I'm installing one regular 9325 AGA bulb and am placing a 40W actinic in the other slot. This is far more interesting for viewing, and much closer to the deep blue color that Lake Malawi has. Since you have a 50-50, I'd consider using that!
Temp often has little bearing on color though. Though they often manufacture the colors in the bulbs to correspond with higher temps, the 5000-6500 range can be very different in color. Some 5000 bulbs peak in the red and yellow, as they should for plants, and some in the blue, green etc. I've got an 18.000
K Aqua Glo (not that great) and a Triton bulb (really good, no temp specified) on my planted tank.
I think the 50/50 sounds good, but just in case you want something slightly different... When buying a bulb, check out the color range on the side. Bulbs that peak in the blue are probably most appropriate for your tank. Or, consider getting a small actinic bulb to complement the light already in the tank, as that will skew it towards the blue, also.