It is definitely okay to add another darker color on top. Make sure you use a permanent color so it doesn't fade and look the same within a few weeks. When you use a color that is darker than your current or natural color, it is depositing color only so it is much less damaging. When you're going lighter, you have to actually change the properties of the hair and strip them of their color so it can be very damaging. So because you're going darker which is deposit only, it won't be too damaging. But because you're a frequent color-er I would also recommend that you start using a deep conditioning treatment to help keep hair healthy and damage free. :o)

Yes, but I would wait at least 3 weeks

yeahh its safe :)
ive done this before. and it didnt damage it or anything x

You can do it, but you should use a semi permanent color, it is less damaging, just make sure it's as dark or darker than the color you have on your hair now.

When you lighten hair you are stripping out the melanin. The reason it turned light brown is that you needed to first "fill" the hair. This is usually done with a semi-perm color with warm tones. Ideally the filler would be orange. It IS safe if you have a semi-perm color. Do not use permanent. Try to go a shade or 2 darker than you actually want. It is going to fade quite a bit in a week or less. If the color you have is a semi but it is the goal shade you can still go for it, just be aware that it will fade. Don't wash your hair for about 48 hours.

Yeah it is

Yes you can. I've done it before several times (with both permanent and demi-permanent). Use a demi-permanent color. Demi-permanent colors just deposit color into your hair and they're less damaging than permanent colors.

yeah its safe.
but from experience dont add another dye on top the day after u dyed ur hair. ur hair wont grab the dye and it pretty much everything rinses off. try to put another dye 2days after/un washed hair.

or use a semi perm. hair dye-that 4sure will make ur hair darker.but leave it on for 10mins and now longer since it will make ur hair darker the longer it stays in ur hair. :)