Acrylic inks can't be beat for pigment intensity, but their colour range tends to be pretty limited, and intermixability can have unexpected results. An alternative, especially for relatively small quantities of wash, is a good quality artist grade watercolour. Pan or tube, it makes no real difference to the outcome, though tubes are easier to handle if you need a lot of paint.
Just mix the watercolour wash to the intensity you want, add some liquid matte medium (I use Golden) and a drop or two of flow-aid, and you have a wash of any colour you like.
If you want a supremely "fast" wash with zero surface tension that will wick right into seams and creases, you can use a 50/50 water/alcohol mix (either meths or iso, either is fine). Be cautious with an alcohol wash though, as it can soften underlying acrylic paint, so it's safer for a pin-washing technique than for slathering on in large quantities.
Watercolours are usually pretty transparent, as the whole point of watercolour painting is to allow light to penetrate through the paint and then reflect back off the white paper beneath. This is the quality that gives good watercolour paintings their vivacity, and is the reason why it's so important not to over-work your watercolours.