To send a color filled shape behind text in Excel, follow these steps:

1. Insert the shape by going to the Insert tab and selecting Shapes.
2. Choose the shape you want to use and draw it on the worksheet.
3. Fill the shape with the desired color by right-clicking on the shape and selecting Format Shape.
4. In the Format Shape dialog box, go to the Fill tab and choose the color you want to use.
5. Click on the Close button to close the dialog box.
6. Click on the shape to select it.
7. Right-click on the shape and select Send to Back.
8. Your shape will now be behind the text.

If the Send to Back option doesn't work, you can try using the Bring Forward option instead. To do this, right-click on the shape and select Bring Forward. Keep selecting this option until the shape is behind the text.