You ask a wonderful question - thank you for the incentive to share my most loved techniques. I must be a frustrated elementary school student, because I LOVE to color in my images! I will preface what I share with that I am a former SU demo, so have no personal stake in promoting their products. When I suggest SU it is because I truly believe their products are the best.
SU Basic Black and Basic Brown are great for stamping the image so that any watercoloring technique you may use later will not run. Embossing is wonderful for that as well (especially if you are using the bleaching technique to bleach out areas inside the stamped image).
SU Pastels/chalks are truly wonderful and give a subtle, soft effect. I usually use a q-tip type makeup applicator to apply the color.
Blender pens and aqua pens using ink are great too, and give a more subtle watercolor look than the markers.
The markers are great for dramatic bold images. I have found for subtle detail work non SU markers in the darker colors (especially greens) give me more possibilities. I use my soft subtles color family the most for SU Markers.
Artists pencils - I love my pencils, and use Prismacolor (can be individually replaced as I use them up!). I haven't used the SU artist or metallic pencils, but many people love them!
Watercolor pencils are some of my favorites. SU is a great place to start for a relatively low cost set. I am a gardening gal, and love to use watercolor techniques for botanical images, so I required much more variety of greens than the SU allows. I bit the bullet and purchased the largest set possible of artist quality watercolor pencils - Albrecht Durer (from Dick Blick and ASW - artist supply warehouse - both have excellent sales, and the individual pencils can be replaced 1 at a time - a real plus for me!). I use a Niji waterbrush to apply water (SU version is the aquapainter, which I've yet to try). I'm a clutz, so having the water in a well within the brush itself is a real mess-saver for me!
Of course, I HAVE to tell you about coloring images in Twinkling H20s! They are great alone or in combination with other coloring techniques. There have been numerous discussions on H20s already so I won't repeat them here, but share a couple of links to them instead:
Forums at Splitcoaststampers
Info on how to use:
Forums at Splitcoaststampers
I'm sure you'll get LOTS of other suggestions as well! I hope you find "coloring" as much fun as I do!!
All my best,
Coloring Geek Nancy in Rockland Co. NY