Here's a second go, this time setting all borders in the three rows to "No border".
Unfortunately, this method does produce thin white lines that I was able to see in the PDF (lower image).
In Numbers:

Printed to PDF:

"I find it odd that Numbers would prevent you from creating solid color blocks without lines."
The colour blocks are 'solid' within each cell, but there are four cells in each row above. Each is a separate element, and is coloured separately, as are the borders between each cell and its neighbours.
Here's a third go, this one using merged cells, all four in rows 1 and 2, the first three in row 3, with the fourth left separate to reserve the position of "Cost."
Row 3 was left unmerged. Numbers would not let me merger three cells in this row, and I did not explore further.
In Numbers:

Printed to PDF:

In general, my recommendation regarding merging cells is "Don't do it."
Merged cells prevent sorting the table, and have had other negative effects reported. They are seldom necessary.
Should merging the cells produce the result you want, you need to be aware that this can cause issues that may outweigh the benefit. Keep your eyes open for possible related issues, and b prepared to back out gracefully if and insurmountable ones arise.
Regards,
Barry