Mixing red and green should be avoided. There’s no problem using either alone. Human color vision is the least sensitive to blue light, so a blue indicator led has to be made brighter than an equivalent red or green led to be as visible in bright ambient lighting. But that makes blue leds disastrous in low light, where the opposite is the case (vision is the most sensitive to blue). Of course there never was any reason for blue standby lights except the fact that blue leds had novelty value and looked futuristic compared to boring old red and green leds.