I’ll repeat the fundamental tip posted earlier by Lee Wilson.
Q: Is there a way to use different images for the instrument (EXS, ES etc.) slots than to audio FX slots following them?
A: Sure. The MAResources file defines what images are used and where. By default Logic uses the same image for Audio FX as it does for Software Instruments… but if you want two different button images (presumably you just want two different colours) all you would have to do is this…
1) Find the file mixer_insert_audio.tif in the Resources folder and make a duplicate – rename this duplicate mixer_insert_instrument.tif.
2) Open mixer_insert_instrument.tif into something like Photoshop and change the colour (Hue and Saturation is the easiest way to do this) or paint on it or do whatever you want to it… then hit save.
3) Now find MixerInstrument in your MAResources file and direct it to your new mixer_insert_instrument.tif file rather than the mixer_insert_audio.tif file.
Notice in the screenshot below how MixerInstrument and MixerInsertAudio both reference the same image file. All you need to do is to point MixerInstrument towards your new file: simply double click on the file name and replace it with mixer_insert_instrument.
4) You’ll have to do the same process to the other uses of “MixerInstrument” (they are right underneath the main one) – otherwise if you do something like switch between the narrow and wide mixer layout the button images will change.