YES. Anyone who tells you that you can't mix it or it will damage something has no idea what he's talking about.

DOT 3, 4 and 5.1 are all fully compatible with each other. DOT 5 fluids are the odd ones out, but you won't commonly see them unless you're riding a Harley or so.

DOT3 is the minimum standard for the brake fluid you can use in your car. DOT3, 4 and 5.1 are all glycol based fluids and can be freely mixed, with the caveat that if you mix expensive DOT5.1 fluid with DOT3 you can't expect the fluid to boil at DOT5.1 temperatures (to put it in the form of a computer analogy - if you have a PC using DDR3-800 RAM and you put in a stick of DDR3-1066 you can't expect the old RAM to run at the higher speed)..

The colour of the fluid doesn't really matter; most brake fluids are almost colourless or they might have a slight yellow tinge. Manufacturers sometimes add dyes to the fluid so that its easier to see.

May I ask why you're topping up your fluid though? That should be one thing you don't need to top up unless you have a leak! As your pads/shoes wear it is normal for the fluid level to drop (the pistons in the calipers/drums keep moving towards the rotor/drum hat to take up the wear). If your fluid is near the minimum level, it's quite likely that you need to change your pads. When you do change your pads, a fluid change isn't a bad idea.

Sorry, saw your PM before this reply.

Check your pads, change them if they're worn (they probably are) and have the fluid changed. Brake fluid *must* be changed at least once every two years as the glycol base in it absorbs water. Brake fluid which has absorbed water will have a reduced boiling point, which you will find out very quickly if you use your brakes very heavily - think of going down Genting.