Clear terrain having two colors is a bit confusing to me, so I made a mod using one color only. I share the mod in case someone has the same need. The pic shown has also applied the following two mods which I like very much -
The mod file : just backup and replace the file - [WitE2 directory]\Dat\GenData\w_hexart.dat
Note: my mod is not a change to the hexarts, but change to the database telling the program which hexarts are to be used. The mod will only take effect on new games.