It is kinda tricky. Even if the person who made the casting would be an android perfectly free of any conscious or unconscious bias, and would honestly just choose the most talented person for each role, the bias would still exist because of the pool of available people to choose from. Other people choosing actors certainly have biases, and this affects the actors' careers, so if you are looking for actors with some established acting chops, the white males will already be over presented in that pool of people. Being 'colourblind (or 'genderblind') this way works only if everybody does it, and they certainly don't.
I've been a casting director. A huge part of that job is just policing the unconscious biases of your bosses -- I've worked for many, many people who had the best intentions, super open-minded liberals in their real lives, but they keep tripping themselves up unconsciously. This actor just doesn't quite FEEL like a lawyer, they can't put their finger on why, but they just don't feel right somehow (it's a dark-skinned black person)... but hey, look at this woman! She just has that upscale lawyer quality more, don't you think? (it's a super light-skinned black person) I could sit here for days and list endless examples of this kind of thing. I've learned I have to almost trick my clients into not making racist/sexist/etc casting decisions... early on in my career, I would challenge these biases directly, and it would always fail and we would end up with the the most straight-white-male casts you've ever seen. When I started approaching everything with a much softer touch and trying to cajole from other angles and almost never mentioning race/gender concerns (while keeping them in the forefront of my mind), I had much better results in assembling diverse casts.
Another challenge in this area is that the pool of white actors is waaaaaaaaaaay larger. You always end up in these horrible moments where you've whittled it down to your top two choices, one white, one not. The whole creative team is assembled, reviewing the tape, discussing the pros and cons of these choices. The white actor is slightly better, because the white actor has worked far more, because there are just vastly more roles available for him. I know the non-white actor has an incredible performance in him, though, if he would just get the chance... all he needs is to actually get the job and settle into it and then he'll wow everyone. You just need to figure out how to get these skittish producers over that hump, because they get stuck in: "I'm looking at these two audition tapes, and the difference is slight, but from what I can see in front of me right now the white guy is better, and I want whoever I think is better even if it isn't a huge gulf between them."
Basically, it often takes an enormous amount of energy, dedication, and subtle persuasion on the part of the casting director to overpower unconscious racist/sexist/homophobic/etc biases in the casting process. It can be tempting to say, it's 10 PM and I'm still in the office, let's just give them what they want and get home.
But back to Discovery... I keep going back to how we don't know who among these actors is going to really carry the series. If our most prominent characters end up being the first 7 announced -- a black woman, an Asian woman, a gay man, and four aliens -- I don't imagine anyone will feel it is insufficiently diverse. Maybe Saru will even be genderless, giving us a perfect 50/50 split!
