A year or so ago I complained about a yellow gradient. Result Gradients were improved, but as a result they affected the yellow on the UI. It isn't my intention and it reduced visibility.
https://github.com/aburch/simutrans/blob/68afb0ef88d66289466015365201f40268f7ba6f/src/simutrans/simcolor.h#L40
#define COL_YELLOW 173I think it will bring back the old visibility.