Each time I create a new notebook, I have to set the magnification to 150% and modify the output color to blue.
Is there a way to automate this using stylesheets or other features?
You can modify default magnification very easily.
Open a new notebook. Open the Option inspector (Format->Option inspector). Select "Global preferences" in the first dialogue box, the move to Notebook options, then to display. There's an option here called Magnification which can be set to what you want. Apply, quit, then your new default magnification should be visible when next you start.
What do you mean 'output colour'?
You can modify a style sheet to have whatever (ish) style you want.
Go to Format->edit stylesheet from the menu. Choose an appropriate style to modify from the list.
Stylesheets are not documented particularly well. David Park has a guide for v6.0 that's still relevant today. There is also a more recent discussion in Mathgroup that covers installation in MMA8.
There's a comment form John Fultz in MathGroup that "The font for StandardForm Input and Output cells comes from the StandardForm style. Similarly, the font for InputForm cells comes from the InputForm style, and for OutputForm cells from the OutputForm style. There is a comment in Core.nb in the group that contains the Input/Output styles which mentions this."
Regards,
Dave.