Get matched with top sand and rock delivery pros in Node, WY

Yes, you can put new mulch over old mulch, since it’s decomposable. The only reason you should remove old mulch before laying new mulch would be to maintain a certain aesthetic. 

Generally, you want the soil for your edible plants to have good drainage, organic matter for nutrients, a loose texture (like loam soil), and a balanced pH level. However, the type of plants you're growing will dictate the exact characteristics your soil needs to thrive. For example, brussels sprout prefers more acidic soil, while broccoli prefers more alkaline soil. 

It depends. High-end colored mulch can be fairly costly. However, you'll have to replace less-expensive natural mulches, like pine needles and cedar mulch, more frequently. 

The cost is balanced out somewhat, though, as although you'll have to replace colored mulch less often, you'll have to replenish nutrients in your soil more frequently to make up for the decrease in organic matter and nitrogen from the mulch.

You should add 1 to 4 inches of mulch, depending on your garden type and needs. Keep in mind that no amount of mulch can completely prevent weeds, but a heavier coverage (closer to 4 inches) can suppress weeds enough that you are only seeing them sprout every few weeks. Finer mulch prevents weeds more effectively than large mulch, so if you’re using mulched leaves or shredded bark, 1 to 2 inches will likely suffice, while 3 to 4 inches is best for larger mulch, like bark chips or straw.