^^^^
I bet no rice makes a color impact. Colored malts do it by contributing Maillard and caramelization products from malting. Corn doesn’t make a beer more yellow than rice, IME. But it would be interesting to hear your results.
What I do imagine would be interesting is trying to get different flavors from, say, Basmati rice, Jasmine rice, etc. Just like corn is a much more flavorful adjunct than plain rice is. Anybody have experience with this?
I’ve had beers made with both Jasmine and black rice. The Jasmine had a flavor impact and the black contributed color.
I’ve had a very nice PrePro Pils in ABQ, made with native blue corn. It was the same color as any other PrePro Pils. I have no idea why some color didn’t come through.
As to using instant adjuncts, the same holds for oatmeal - a well known brewer I know uses Quaker instant breakfast oats with no problems. He has won many awards for his oatmeal stouts using instant oats. Just thought I would mention that for the OP.
I use quick oats from the bulk bins at the grocery store.
Okay, Quaker instant grits instead of flaked maize, anybody? I don’t know how that’s processed, whether it’s precooked or just finely ground. (Or if I’m really that cheap. 
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I did a cream ale with Jasmine rice but didn’t get around to doing the same beer with flaked rice so I can’t know if there’d be a noticeable difference. As for black rice, when cooking it definitely turns the boiling water into black ink so Denny’s experience isn’t surprising, and I wouldn’t be surprised if it affected flavor as well. Black rice and black beans, to me at least, are way out there in worlds of their own when compared to other types of rice and beans.
As for oats, I just use regular rolled oats. Are instant grits any cheaper than flaked maize Rob? Not being a fan of grits I’ve never priced them.