Hair porosity?

What kind of dye are you using? If it's a direct/veggie/true semi dye it's not going to make a massive amount of difference in the end as they're more of a stain than anything.

I will be using Directions! So from what you're telling me, I don't have to worry about taking any preparations regarding porosity to dye my hair, right?

I think it depends on what colour you plan to use. If it is a pale colour or a pastel, it can show up the differing porosities in your hair. My hair as an example, holds the colour much better at the first 6 inches, these parts actually look darker when freshly dyed. The mid lengths fade fastest, the tips tend to fade differently, the pink disappears leaving just the blue tones from the lilac dye.
You can even out the porosity by doing a protein treatment before dyeing. Avoiding damage from over processing and heat styling makes a huge difference.  Porous hair is damaged hair, so keeping your hair as healthy as possible will ensure the most even results.

It really depends on the hair. Like, usually curly hair tends to be more naturally dry and porous, but not always. Natural texture or damage can play a big role into how much dye is absorbed and released too, but everyone's hair is different. More porous hair, like hair that has been bleached to platinum, then say you want to dye it a permanent light brown, will usually come out a medium or even dark brown at first but fade back to a light brown or even lighter.

With semis it doesn't make too huge a difference in the color outcome, it can go darker if the hair is more porous, but it can also go lighter too. Sometimes it really sucks color up, sometimes it doesn't want to take the color as much.

Doing a protein treatment a day or two before can help even out porosity and fill in any gaps in the hair, giving you a more even result. If you are worried about it coming out too dark though, you can do a test strand with the color used neat, and one diluted 50/50 with conditioner. The good thing about these dyes though is they aren't damaging at all, so if it's not dark enough you can just go over it again, and if it's too dark, just wash it a couple times with a sulphate/clarifying/antidandruff shampoo 🙂

These dyes can bleed/transfer a lot depending on the hair/dye, adding a good squirt of conditioner to it will help with that, and really doesn't affect the color much, if at all, especially with darker colors.

Once you've applied the dye, the longer you leave it on usually the longer it will last. I like to leave mine on for 4 hours, any more than that and it doesn't seem to make much a difference in staying power though, but less than that and I do notice it fades faster. After you apply it to clarified, non conditioned, product free hair (some apply it to damp, some apply it to dry hair, it's all preference. I always get a better more even result when it's dry though), put a shower cap/plastic bag/cling film over it, and you can warm it up with a dryer for 15 mintues to help open the cuticle more, letting more dye in. When you're done rinsing it out (some people shampoo, some don't - I say if the color run off is really dark, give it a quick shampoo to help remove excess dye - it will help get it off your scalp too) and conditioning, spray or pour some regular white vinegar diluted 50/50 with water over your hair to help close the cuticle and keep the color in.

For upkeep, use a sulphate free shampoo (helps a lot!), or have a search into CO washing - it helps tremendously and is very good for the hair. Stretching your washes/co washes helps, you can add some of your semi to your shampoo/conditioner too. The sun, swimming and heat styling can fade color too. I found that heat styling fades pastels a lot especially, but doesn't make too much a difference on darker colors.

What color are you planning on using? Can't wait to see how it goes!

ETA:
just realized we have a thread that explains all this stuff already lol. It may be repetitive from what I've already said but I'd definitely give it a read:
http://www.hairdyeforum.com/index.php?topic=11009.0

Oh wow, thanks so much for all the help guys! I am poring over all the information provided and formulating my  plan of attack. As for which dyes will be "attacking" my hair, I have Directions Plum, Midnight Blue, and Rubine coming to me soon. I'm not sure how exactly I will be using these colors, but I have five bundles of hair trimmings that will be tested once the dyes arrive.