You may find that when your build output for a step reaches a certain size that the colour output is disabled for the whole build output.
This is an intentional feature which disables colours once the output reaches ~1000 lines for performance reasons.
Beyond this number of lines, the performance of the UI/build output can start degrading beyond an acceptable point which negatively affects the user experience.
This is the same reason why we do not display over a certain number of lines in the UI if the build output becomes too large.
It is not possible to override this default behaviour at this time.