this is currently not possible. The Dashboard and Custom Chart features are currently undergoing improvements though and the ability to change colors for custom charts is on the near term roadmap as you can see in this thread: https://answers.dynatrace.com/idea/181318/planned-...
There is also a mockup of how this will probably look like:
Great functionality - but apparently the color is linked to the charted entity, and not the metric ...
When we add more than one metric to the chart for a single service - then they all show in the same color. Sometimes separate colors per entity/metric combination would fit better.