There is another artist Koolen. This is very coincidental. The other Koolen (Harry or Harrie Koolen) was a painter and art teacher and lived from 1910 to 1990. He painted mostly scenes of villages and towns, flower still lifes. The style is quite different and easy to distinguish.
Nevertheless on the Internet it is sometimes confusing and often paintings are offered signed with (H....) Koolen which are not made by the portrait painter Koolen (1904-1985). These works are - in good faith - made by the painter Koolen (1910-1990) or unfortunately sometimes by others with fake signatures.
Just for information some examples: