I think that the result is correct, because -4142 means xlColorIndexNone (https://learn.microsoft.com/en-us/office/vba/api/excel.xlcolorindex), i.e. no specific colour is assigned yet. The myCell.Interior.Color returns 16777215, or FFFFFF using Hex(myCell.Interior.Color), which denotes the white colour. In case of blue background, for example, you should get FF0000 (FF is Blue, 00 is Green, 00 is Red).
Make sure that you are checking the right range.
To get the text colour, try myCell.Font.Color.