By default the color of content shown in toolTip is taken from dataSeries color and it seems to be working fine, please take a look at example in this documentation page.
If you are still facing issue, kindly create sample project reproducing the issue you are facing and share it with us so that we can look into the chart-options you are using, understand the scenario better and help you resolve.
From what we have observed, sometimes things get delayed mostly when we are not able to reproduce the issue or not able to understand the exact requirements or the solution that we provide may not work properly due to the variation in chart-options being used by you and us.
Having a sample project helps us in figuring out the issue and many a time we can just edit your code to fix the issue right away.
—
Vishwas R
Team CanvasJS