Grey is a thing. One of Paul Downing's books actually has a diagram going from black to white you place your stone next to and compare. Grey is more valuable than white but less than black.
1. Do not grind through the color bar. Looks like you have some really nice rough(and you are taking better pics of opal than I am currently). If you are talking about the last pic with the stone standing on end, do not grind that down. Your money is in the weight of the stone. That looks like a fairly decent color bar and you want to save as much as possible.
2. Remember you are working opals, not making cabochons. You aren't trying to make a big dome. Its a pain in the butt for me to switch mind sets when I go back and forth from opals to traditional rocks because the process is similar yet very different. I try to remember when I switch back to opals to do a practice opal first to get bact in the mindset. You want just enough dome to polish. That's it. Stones look almost flat with a barely perceptible dome to them. The idea again is to minimize what you grind off to maximize the weight. SEDA opals has a video they took when they were polishing a black opal and you can hear one brother telling the other brother who was simply trying to clear a tiny bit of potch, "You know each time you do that we are losing a hundred dollars right?" "Yeah I know." And then he repeats it several times as the brother does very light swipes over the wheel. You can tell he is getting aggravated although the end stone was amazing.
3. Even small opals are worth messing with. I made a really shiny one last night that was half the size of a pencil eraser. It will look good as part of some combination jewelry I am sure when I get around to making it. If anything its good practice, and even better its really quick and time efficient. I spent maybe 5 minutes getting it from the 260-3k wheels last night.