[Phanx]

You will most likely have to use secure hooks. For an example, here is the function I use in !ClassColors to apply custom class colors to the health bars of the default raid frames:

hooksecurefunc("CompactUnitFrame_UpdateHealthColor", function(frame)
    if frame.optionTable.useClassColors and UnitIsConnected(frame.unit) then
        local _, class = UnitClass(frame.unit)
        if class then
            local color = CUSTOM_CLASS_COLORS[class]
            if color then
                frame.healthBar:SetStatusBarColor(color.r, color.g, color.b)
            end
        end
    end
end)

You'll need to change the function, object, and table names, but otherwise it should be pretty similar. You can't modify the original table at all, so you'll need to add a bunch more hooks to apply your custom colors to power bars in other parts of the UI as well.