I've dyed a lot of clothing, and I can already tell you, novice or not, results are never guaranteed to be perfect. However, prepping your garment is 99% of the battle.
This is how I prep:
I get the garment under bright, natural, midday sunlight, and I look for stains, or already uneven coloring. I really take my time here, especially if I'm dying a light-colored item a darker color.
If there are stains, I work to remove those first - especially underarm stains. (I love Carbona Stain Devils Rust Remover for underarm stains). Stains on a garment, will inevitably dye darker than the rest of the garment, in my experience.
Bleach marks, on the other hand, don't hold the dye, and come out lighter - again in my experience. I don't know if there are workarounds for bleach marks.
My go-to dye is RIT, and they have an entire, amazing catalog of color forumulas. I've successfully changed mid-blue jeans to deep blue that has a hint of green, and my all-time favorite dye project, a white Theory button-down blouse that I thrifted, and re-dyed a pale peachy-pink color.
So let's talk about changing the color on an item that is already dark, like your black and gray item...
If the item is already black: 've never successfully redyed black to any other color except black (I do this to refresh beloved black pieces).
If the item is gray:
The best way to approach this, is find a piece of fabric in a similar shade and fabric makeup, like an old tee-shirt, and test out your color formula on that, first.
One of my most successful gray-to-color projects, was a J Crew cotton-blend sateen pencil skirt, that was a pale dove-grey color. I thought the fabric was beautiful, but realized I wasn't happy with the color, so I used Dylon in purples and blues, to get it a deep, vibrant violet color. I eventually sold the skirt on eBay. (I didn't get a photo, but I've posted a flower photo below to give you an idea of how it turned out - most like the purple in the lower right corner.)
I tested my color formula by using an old mid-grey t-shirt. I cut it into strips, and kept dipping, and then tweaking the formula. (I think I had to add more purple to color, correct, but I don't remember).
I've dyed a pair of J Brand jeans from a very average blue, to a richer, darker green-based blue, that I loved.
I tried dying a mid-charcoal Wolford skirt and jacket to a deeper green-blue color. It wound up being a more blue-based charcoal, which I'm satisfied with. I didn't full out hot-water dye it, because it was a wool blend, I didn't want to risk ruining or felting it. I think hotter water would have affixed it better. I dyed this in my stainless steel sink, periodically adding boiling water a little at a time, so that the water was hot, but not actively boiling the clothes.
I may eventually try this again, but I'll go hotter with the temperature.
So let's talk about dye options: when I lived at my old place, I used my ancient washing machine. After dying anything, I would run a hot water wash cycle with blue Dawn dish detergent, and a couple cups of bleach. Then I would do a second full wash cycle, with just water. I never had issues.
Now we have a fancy front-load washer, and because the item isn't submerged fully in water, I would use a stovetop method (which my pro costumer friend swears by).
I've done stove-top dying, and it's pretty easy, but way more hands-on than dumping it in the washing machine. You need to use a pot that you designate solely for this purpose, that you don't ever use for food preparation again. I have an old stainless steel pot, and an old nylon spoon, and a yardstick, for stirring and manipulating the garment.
TIPS:
Put down old towels in your work area
Wear gloves
Blue Dawn Dish Detergent and/or Head and Shoulders Shampoo is great for mopping up spills on counters and floors, but nothing is 100%, so if you spill, move fast!
My dyeing toolkit:
White vinegar, and Salt, to help fix the colors
Yardstick and nylon utensils, for stirring (dollar store)
Heavy Stainless steel pot (hit up your local thrift store)
Zout stain remover, and the Carbona Stain remover line, to pre-treat clothes
Blue Dawn
Old towels
Old tees, cut into strips, to test your dye color mix, if you're trying a custom shade
PATIENCE and A SENSE OF HUMOR.
