My understanding is that everyone from a specific country essentially needs to wear matching kits in ITU. Read the following rules below from Triathlon.org (direct link here):
4.1. Uniforms must be in the colours chosen by the National Federation (NF) for ITU
World Championship Grand Final, ITU World Championship Series, ITU Continental
Championships and ITU Multisport World Championships events. For all other ITU
events the uniform colors have no restrictions.
4.3. It is intended to give the country uniforms a distinct look. However the design
definition should not impact on technical requirements that athletes might have due
to body shape or size.
If you skip down to Appendix C of their rules, it also states:
- General Requirements:
3.1 Every NF uniform has to be different from any other NF uniform, according to the
ITU Uniform Rules. The difference will be significant to avoid confusion between
two different NFs.
3.4 Every NF can request the approval of a different uniform design for every different
multisport, and two different uniforms for triathlon, indicating which is the 1st option
and which is the 2nd option. However, all athletes from the same NF, competing in
the same event, will wear the NF’s same distinctive design.
I think the part I underlined directly answers your question.