Hi Vicki,
You are right, shading with the lighter colors is a bit easier... the darker you go, the easier it is to overpower and get a "one-shade" look.
I know there are other techniques and tips out there and hopefully others will share, but this is what I do...
I always start with a shade WAY lighter than I want the final look to be. For example, if I wanted a dark red shirt, I would start with R22... that is sort of a light salmon color. I then add R24 (medium red) to MOST of the shirt, leaving the light color only where I want highlights. Then I go in and add a darker color R27 maybe and add that to part of the shirt covering MOST BUT NOT ALL of the R24 color. By now my shirt is mostly the dark color with just a touch of highlighted areas...but I still want it to look "realistic" so I need to add shading... and to add my shadows, I go in with a super dark color R29 maybe and add just a touch to the areas I want shaded.
So in essence, I use the same technique that I would for any other color/shade of shirt, but when I want to go really dark, I just add an additional layer of color... so instead of using the typical 3 shades of red I would use 4. It's more work... but the results can be really cool! (I've even got one image where I used 6 different shades of the same color to get LOADS of contrast.)
Using the colorless blender to add highlights is good... you may even want to try adding them with an opaque white ink for even more contrast without washing out the shirt.
Hope that helps.
Happy Coloring!