I am no expert on this, but oddly I was doing some experimenting today with the same lenses as you. From what you are describing, two things come to mind. And if I am wrong the experts here can correct me and I learn along with you:
1) The yellowness in the second photo appears to be from tungsten lighting? If you set the WB to correct for that, it goes away.
2) Are you talking about contrast and not absolute exposure? The A 50mm will have much more contrast than the zoom.
I took the following photos today. ist DS, same camera settings for each, ASA 200, f8, and 1/125 sec. The two zooms were set to 50mm. They were offhand and not mounted. Jpg mode and not altered.
1) Pentax A 50mm 1:2
2) Pentax DA 18-55mm
3) Hanimex 35-70mm