Yes, you can set the color of the symbols based on a Z column. Part of the color properties includes opacity. So if you want the symbols to be the same color, you can keep the color the same and only adjust the opacity. For example:

1. Select the Post layer in the Contents window.
2. In the Properties window, click the Symbol tab.
3. Under the Symbol Color section, set the Color column to be the Z column for your data (or the column from which you want to vary the color properties).
4. Set the Color method to be Numeric via colormap.
5. Then click the (…) button to the right of Symbol colors.
6. In the Colormap Editor dialog:
   1. Select the node on the left (the minimum value) and then click on the color you want the symbol to be.
   2. Then click on the node on the right (the maximum value) and then click on the color you want the maximum symbol to be. This could be the same color as the minimum.
   3. To change the opacity (for Surfer 15 and below see opacity instructions below):
      1. Click along the line in the Opacity Mapping section to add a point.
      2. Click again to add another node, then click and drag or enter a Data value to position the 2nd new node above the 1st node:
         
      3. Select the first new node and type a new value in the Opacity field. 
      4. Repeat setting the opacity as needed. 
7. Click OK and the color and opacity is applied to the symbols.

To change the opacity of individual nodes in Surfer 15 and previous follow steps 1-6 above then:

1. Once in the Colormap dialog, select the node on the left (the minimum value) and then click on the color you want the symbol to be.
2. Uncheck the box next to Apply opacity to ALL nodes. 
3. With the left most node still selected, type a new Opacity value. 
4. Then click on the node on the right (the maximum value) and then click on the color you want the maximum symbol to be. This could be the same color as the minimum.
5. If necessary, adjust the Opacity for this node as well. 
   
   

Updated September 18, 2020