Unfortunately, it's a built in feature. To get a different highlight color requires using a different text color.
Do you really need to select the portal row? (Done by clicking an empty space in the portal row).
Clicking into a field makes that portal row "active" and doesn't highlight the row, so that might be an acceptable work around in some cases.
You can stop Users from selecting 'the portal' itself this way:
Create a rectangle. Color it so you can easily see it. Remove the lines if they exist ( Inspector > Appearance and select 'line' and 0 pt). Measure your entire portal row. You will have to change it temporarily to one row to get its dimensions. Make rectangle exact same size and place it exactly over the first row so top and left px match same as portal.
Attach button to this rectangle with script which either 1) goes to the first field (or any field) a User can modify, 2) commits the record or 3) goes to a named object if you do not wish to commit. When ready, make the rectangle transparent and send it to the back (using Arrange) and then send the Portal to the back (in that order) so that the portal is the bottom stack and then the rectangle and the portal fields are on top. If User then clicks anywhere but entering a field, the portal will flash but not stay black.
Thanks everyone for the great tips. There seems to be a wealth of valuable experience on this message board.
It turns out that my issue was that I had a script step that sent the focus to the first portal row (go to portal row [select;first] that was the culprit. I have the entire portal row covered with edit boxes which have the "browse" unchecked so they can't be entered. This solves the problem of the actual portal row being selected.