What do users remember the most, position or color of elements?
I will not come up with concrete suggestions, but a short comment on the headline: it is highly individual. Just like some people will say "I see what you mean" and others will say "I hear you" or something similar. Some people are tuned to visuals, some people are more tuned to sound.
Some people are more attuned to shape, others to colours. Just as you need to satisfy colourblind, you need to take into account that not all people see shape-and-colours as their primary clues to navigation (some people will identify a tree, an animal, a house by shape or colour; some will identify a car driving by by sound rather than shape and/or colour etc).
In this instance, it is of course a balance between the two (it would be fun to find out what kind users blunder into the wrong button. There might be a pattern there. Also, I think users with excellent computer skills will be faster at assuming where to find the right button.).
Since you cannot show us the actual buttons with the icons, it becomes difficult to suggest. One simple way could be simply to make the distance larger to the one "out", as @DA01 suggests. Sort of like MailApp:
Just a thought.
One experiment that is fun, is to present a handful of people a primitive mockup (The simpler the better: drawings, post-its on a wall, flip-over sketch, whiteboard etc) with buttons placed but not marked and identified. Then ask people what they expect the anonymous buttons will do. Sometimes, this can bring informative and very interesting results.