Generally speaking, it is perfectly safe to mix one colour with another but it is advisable to mix only ink of the same brand together. It is good practice to always mix into a new, clean container and check for any solids that may precipitate as a result of any chemical reaction although this is rare. Remember: ink is relatively cheap, pens are not!
We stock over 400 ink samples which enable you to try mixing inks in small volumes at little cost. Our empty sample vials are ideal for mixing inks, coupled with a filling kit you can use the syringe to dispense just the right amount of ink into each vial to create your custom ink. Don't forget to make a note of your ink recipes!
For inspiration artist, Jane Blundell has produced a fantastic resource for mixing De Atramentis document inks. The same procedure can be used for any ink brand.