Document deterministic colors of nOS-V task types

doc/nosv_type_labels.md

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+# nOS-V task type colors
+
+The color assigned to each nOS-V task type is computed from the task
+type label using a hash function; the task type id doesn't affect in any
+way how the color gets assigned. This method provides two desirable
+properties:
+
+- Invariant type colors over time: the order in which task types are
+  created doesn't affect their color.
+
+- Deterministic colors among threads: task types with the same label end
+  up mapped to the same color, even if they are from different threads
+  located in different nodes.
+
+For more details, see [this MR][1].
+
+[1]: https://pm.bsc.es/gitlab/rarias/ovni/-/merge_requests/27