Apple computers come with a built-in app called Digital Color Meter. If you have a monitor that produces accurate colors you can use this tool to identify the exact RGB values of the colors used for any element displayed on the screen, including the built-in columns of the watchlist. None of the standard colors provided by Thinkorswim language match those colors so you have to use the CreateColor() function to dial it in.
The text is extremely small on a 4k display so I was only able to arrive a close match to what is being used for the built-in watchlist columns. Seems to me this is one of those details that folks waste a lot of time trying to dial in. What productive value does this serve? So long as you can tell the red from the green I suggest chasing things like this is a complete waste of time. But I understand some folks are bored and need something to do with their time.
Here is some code that contains colors which closely match what is displayed in the built-in columns:
plot data = 100 * (close / close[1] - 1);
data.AssignValueColor(if data > 0 then CreateColor(0, 195, 49) else CreateColor(237, 87, 87));