If you pass the Custom Product Property column “Colour #269c7e40a54a462e884edb004c5f7bc8" with an empty value in the CSV import and have activated the import option “Replace product properties”, the Color property should also be set empty for the product. That is currently not happening. Only one change (value from red to yellow) is executed. Without the “replace product properties” option, it is only added. There is no way to remove the property from a product (i.e. set from color:red to no color value at all).