Set Color Control using Dropdown Menu Control

You are not supposed to use self-references with "this". You may want to review the pertinent section in the online help:

https://helpx.adobe.com/after-effects/using/legacy-and-extend-script-engine.html

The rest really isn't that complicated. Since expressions will always return the last valid result of a function, you simply slap on your custom colors after all the dropdown code.

if(effect("Checkbox" == 1){use this color}

else {use result from dropdown magic}

Mylenium

As @Mylenium states, there are no event listeners in After Effects Expressions.  I have found a way to work what you want without extra controls, but it involves having a third dropdown property called "custom".  See screenshots below.

However this way doesn't update cleanly in Essential Graphics and I would recommend against using this in a MOGRT, just in case you were thinking of.  AE expressions are limited I'm afraid - I often have to add some extra controls to EG to keep things working reliably.  IMHO I think you're better off adding a checkbox, I know it can feel clunky but in the limitations of AE, I think it's the most reliable option.

This is not possible. There is no onChange() event in expressions and what you want would cause a causality loop/ cyclic redundancy loop that cannot be resolved in the expression itself because it simply does not know what the controls' default state would have to be. This needs to be all included in the expression itself and hard-coded as well as structured accordingly. That's why you have to use multiple controls.

Mylenium

Contrary to what @aTomician wrote an extra control would not be necessary. Expressions evaluate strictly in the order they are written out, so all that would be required to allow a custom color is to place it at the end of your other code. However, of course there are usability considerations here. Without an extra checkbox the user can always override the color regardless of the state of the dropdown and that may not be desirable. So indeed I would add that little checkbox as a safeguard.

Mylenium

yes it's possible, add a checkbox control for a "color override"

expression would be something like: if color override is false, if symbol is correct then color is #0000ff else if symbol is wrong then color is #ff0000

if color override is true, then use color control.

sorry in a rush so can't write out the expression fully but shout if you get stuck.