I have been to the Palouse two times now, once in June and the other in late August. To say the experiences were different would be an understatement. In June, the farmland is all shades of green. When the wind blows, you can actually see the top of the crops moving, as if they headed in the direction opposite of where the wind is coming from. Although you can find the huge and powerful farm equipment here and there, there is not much of it in use. That is because the seeding was completed weeks before which created this "sea" of green across the landscape.
Fast forwarding to late August, everything has changed. The harvest was well underway. Brown replaced green as the dominant color. While many fields still had crops that could show the movement of the wind, more had the look of the farmland in this photo. Where there was an absence of large farm equipment in June, there was equipment everywhere cutting and hauling crops in large trucks. You knew where the the harvesting was happening by looking at the clouds of dust in the distance.
So, you might suspect that June is a better time to visit. On my first day of harvest, I would have agreed with you. By the end of my visit, I had a much better appreciation of harvest time, as I think that my initial reaction was more in shock at the change. I still prefer the spring season, but not by that much. I think that had I visited at the very start of the harvest (which is hard to predict) it might have been an equal match.