I did save the .tck file with the file extension .nii. It opens fine in both AFNI and DSI studio, but it does so as a grey scale image.
In these softwares, are you opening this file as an image, or explicitly as a track file? If the latter, it’s possible that the software is figuring out that the data are in fact stored in the MRtrix track file format despite the erroneous file extension; and even though it ‘works’, I would most definitely advocate changing the file type extension to .tck to reflect the actual file contents. Opening this file as a NIfTI image (as indicated by the .nii extension) should almost certainly fail in any software. I would also avoid referring to the result as a ‘greyscale image’ if this is the case, particularly given the ambiguity regarding whether you’re dealing with track data or an actual image.
I would also check to see whether DSI Studio is capable of importing a text file where an RGB triplet is provided per streamline point, or only a single RGB triplet per streamline. Either of these should be relatively easy to calculate yourself, though the visual results will certainly differ (you can achieve the latter in mrview using the ‘Colour by endpoints’ option).
Rob