I'm not sure you can do that for iCustom. Multi-color indicators usually have multiple buffers, one buffer for each color, so instead of looking for color value look for buffer value, i.e.:

buffer1 is green, buffer2 is red. Usually, they are not diplayed at he same time, say when there is an uptrend, green is displayed, when downtrend, red is displayed. That means, when green has a given value, red is 0 or -1 or novalue.

So, if buffer1 is positive and buffer2 is 0 (buffer1>0 && buffer2<=0), you know there is a green color displayed.