The Scope block supports setting properties for only the first six lines. To set properties for 7th and more lines, you can use newer version of the Scope block if you are using MATLAB R2012b or later.
First, make a copy of your model (let us not change the original). Execute the function "replace_scopeblk(modelName)" on this copy. This will replace Scope blocks in the model with a newer version of the Scope. Run the simulation. Open the style dialog (access using View->Style or right click on the axes). You should be able to change the properties of all the lines on the axes. Play around with the new Scope and if you like it, you may use the script on your existing models directly.
To use the new Scopes when creating future models, save it in a library or another model and copy it from there. Today, it is not present in Simulink library. Use: add_block('built-in/TimeScope','modelOrYourCustomLibName/Scope').