I don't think the feature you are after is supported as yet. What you could do though is add your color expression as a variabler. That would have much the same outcome as adding it to the master library as you would only have to edit it once to make a change regardless of how many places you use it. You can access the variable editor via the little "x=" button towards the bottom left of the page and create a variable, e.g. vExpressionColor, give it the definition of your "pick" statement and then evaluate the variable in the color expression e.g. $(vColor).
Hope that is useful.