I have trouble seeing blue light, especially in the dark - the blue is really fuzzy and hard to see. So I went with purple instead. I got a 10-pack of T10 LED lights for about $6 on eBay and it was a quick remove-replace scenario once I got the cluster out of the car. The T10 bulbs that the 240SX uses in the gauge cluster are just a twist-lock installation so the only wiring you have to worry about is plugging the cluster back into the wiring harness when you're done.
Red and green seem to be popular colors also, since blue is generally more harsh on the eyes. It all boils down to personal preference though - hence why I chose purple. If you like blue that much though, go for it!
