After I ruled out every possible yellow wildflower, I knew it must be a grass. So a search of blooming yellow grasses in Texas led to Wikipedia describing my grass precisely. It's called Greenthread Navajo Hopi Tea (Thelesperma Megapotamicum), and it's pretty common in the central United States.
I dug out of the trash can the image I had originally processed, wrongly assuming that the purple on the leaves was a bunch of chromatic aberration (CA). No. It's just part of this plant, and important.
New (original) image and full description (TMI) can be found here:
I don't know what to say about mustard. But several times in the past few weeks, while researching plants, I've learned that a plant might be known in a community by a particular name, and that is spread. Such is the case with (false) shamrocks. Wonder if that's what happened with the mustard. I've seen this particular plant in other areas, and in abundance. All other leads turned out to be quite rare (endangered) and didn't quite fit. This one fit perfectly.
Thanks for your help. Hope you find this information interesting and helpful in your own searches and processing.
