You might want to look back through the posts here for color images. I've posted a few color combination images, as have a few others.

The basic technique for combining images to color isn't too hard, and you can do it with PaintShop Pro. That's what I use for the simple combinations.

Sounds like you have the arithmetic part handled. The thing you may have missed is on the Colors menu: Channel Combining. In the dialog box you assign images to channels, click OK, and away you go.

I used the red=(L2+L3)+L4, green=L5, blue=L6+L7 recipe for quite a while, but it seems to overcomplicate matters, mostly because the L2 is pretty "out there". L2 pictures of the sundial show the blue chip glowing brighter than even the red chip, and I've heard it explained that the chip responds both to infrared as well as visible blue. Although this quirk of the chip doesn't directly impact images that use L2, it makes judging the calibration using the sundial a lot harder. Also, I haven't seen much practical difference between L6 and L7, so I don't often bother with that combination either.

Nowadays it seems like I can get the best automatic match with the sundial by using L4, L5, and L6. L4 is usually described as "orange", but that's kind of arbitary. By the time you've cranked down saturation by the customary 45%, it doesn't make a lot of difference. Unless you have some reason to sweat the colors, L4_L5_L6 is the way to go.

For comparison, this is a picture of the sundial taken on Earth in the lab:

Hope this info helps, and enjoy yourself!