Here are some quick thoughts:
• Colorists should have a portfolio of page samples showing professional quality coloring. 10-12 pages in your online coloring portfolio is a good sample pool. Only post your absolute best work in there.
• Having different coloring approaches with various styles of line art can show flexibility/diversity that could help show editors/art directors what you’re capable of.
• Colorists are the last part of the art production pipeline so they need to know their own productivity. Unfortunately they’re usually the ones who have to deliver awesome work as a project screeches in towards the final deadline. If you’re not able to deliver under the gun it will be tougher to find work.
• Make sure you understand the file format requirements for print and understand the differences between CMYK and RGB file formatting.
• Comic coloring isn’t necessarily about detailed rendering. It’s about establishing focal points and creating mood. Look to your favorite illustrators/colorists. Carefully analyse how they establish light, volume, mood, and focus.
• Make sure your coloring samples are done using pro quality line art. If the line art isn’t publishing quality than even the best coloring will look unprofessional/sub-par.
• Quite a few professional comic artists post large/high resolution line art image files on deviantArt, so that can be a good place to start.
• Like most comic work, you’ll probably start off working on small/low-pay indy comics or anthologies before referrals for bigger jobs come around if your work is good/consistent.
Best of luck with your creative pursuits!