Step 1:
Copy all of the code from the study that comes with Thinkorswim. The study is named "MACDHistogram". Pate that code into a new chart study.
Step2:
Copy the line of code that begins: Diff.AssignValueColor
Step 3:
Paste that line of code to the bottom of the code you pasted in step 1.
Step4:
For only that last line of code you pasted from step 3, change Diff.AssignValueColor
to AssignPriceColor
Step 5:
Save the new custom study and apply it to your charts.
I wrote this out so that you could see and do the steps required to complete the modification. Having completed it for this study, you will find it very easy to apply this to a large number of other studies. Keep in mind that only one study on the chart can set the paintbars. If you try to do it from more than one study on the chart you will get an error message in the upper left corner of the chart.