I recently bought a pair of MKII GLI Recaros myself, and after reading this and other posts while deciding to dye or not to dye, I took the plunge, with expectations sufficiently tempered. Just thought I'd post my thoughts and some before-and-after photos, for others' reference.
My 76 has navy blue (Marine, I think) vinyl interior, and matching the color proved an elusive goal. I did test swatches of the following three options:
Duplicolor Medium Blue - I'd describe this as a grayish light blue. Nice color, but nowhere near navy. I ended up using two cans of this to paint/dye my old, faded carpet. Looks much better, but I don't think it will last too long. (Even after many passes at cleaning, the carpet was still pretty dingy, so the requisite prep was simply not done. That's okay: it's only intended to hold me until I scrape together enough for one of Esty's beautiful carpet kits.)
VHT Vinyl/fabric - Dark Blue - This is definitely darker, but has a pretty distinct green/teal tint to it. Definitely not a fit
SEM Color Coat - Shadow Blue - Not as dark as the VHT, but much better fit, tone-wise.
To prep, I did as Esty urged and cleaned, cleaned, and cleaned again. My process:
1. Vacuumed first to get all the dust and grit out;
2. Spot cleaned with a brush and some spray-on foamy stuff (not many spots, thankfully);
3. Used a rug doctor upholstery cleaner (two passes: much patience required);
4. Isopropyl alcohol with microfiber towels
5. SEM's plastic/vinyl prep: again, patience was key. Was very tempted to half-ass this step, considering how long the previous steps had already taken.
Then, I taped off the plastic and metal bits and did INNUMERABLE light coats on the seats. This took FOREVER. FOR. EVER. I would do a very light coat, then quickly brush over the the area with the nylon brush, to avoid it getting crispy. (Again, much appreciation to Esty for this tip.) I would do a couple light coats per night before heading to bed, and repeated for 4 nights. I'm quite proud of myself for not snapping on night two and emptying a can into the seats. The temptation was very real. TIP: Keep an eye on your brush to make sure it's not getting gummed up with the spray dye/paint. The stuff coagulates and cures VERY quickly, and if you're not careful it'll kind of crystallize on the brush bristles, severely impeding the effectiveness of your brushing.
I ended up using three cans of SEM, plus the can of SEM Prep.
All in all, I'm pleased with the results. Certainly not perfect, but the color goes with the rest of the interior pretty well. A word of warning, though: even with all the prep and patience listed above, the softness of the seats is essentially gone. That's a real shame, since these seats were BUTTER prior to my futzing. I might try some more brushing, but I expect any gain in softness will be accompanied by a proportionate loss in color.
Here is one of the seats before:
...and installed, on the Duplicolored carpet:
I hope that helps somebody.