Add AR6 category colors to PYAM_COLORS

RELEASE_NOTES.md

@@ -18,6 +18,7 @@ an empty **IamDataFrame**. Previously, this raised an error.
 ## Individual updates
 
 - [#651](https://github.com/IAMconsortium/pyam/pull/651) Pin `pint<=0.18` as a quickfix for a regression in the latest release
+- [#650](https://github.com/IAMconsortium/pyam/pull/650) Add IPCC AR6 WGIII colors to PYAM_COLORS
 - [#647](https://github.com/IAMconsortium/pyam/pull/647) Pin `unfccc-di-api` to latest release
 - [#634](https://github.com/IAMconsortium/pyam/pull/634) Better error message when initializing with invisible columns 
 - [#598](https://github.com/IAMconsortium/pyam/pull/598) Support mixed 'year' and 'datetime' domain

pyam/plotting.py

@@ -77,6 +77,24 @@
     "AR5-RCP-4.5": "#79BCFF",
     "AR5-RCP-6.0": "#FF822D",
     "AR5-RCP-8.5": "#FF0000",
+    # AR6 WG III scenario categorization (C)
+    "AR6-C1": "#97CEE4",  # C1: scenarios that limit warming to 1.5°C (>50%) with no or limited overshoot
+    "AR6-C2": "#778663",  # C2: scenarios that return warming to 1.5°C (>50%) after a high overshoot
+    "AR6-C3": "#6F7899",  # C3: scenarios that limit warming to 2°C (>67%)
+    "AR6-C4": "#A7C682",  # C4: scenarios that limit warming to 2°C (>50%)
+    "AR6-C5": "#8CA7D0",  # C5: scenarios that limit warming to 2.5°C (>50%)
+    "AR6-C6": "#FAC182",  # C6: scenarios that limit warming to 3°C (>50%)
+    "AR6-C7": "#F18872",  # C7: scenarios that limit warming to 4°C (>50%)
+    "AR6-C8": "#BD7161",  # C8: scenarios that exceed 4°C warming (≥50%)
+    # AR6 Illustrative Mitigation Pathway (IMP)
+    "AR6-IMP-LD": "#DAA25A",  # LD: emphasis on a low demand for resources
+    "AR6-IMP-Ren": "#EED2AE",  # Ren: emphasis on renewables resources
+    "AR6-IMP-SP": "#F7E7D7",  # SP: emphasis on sustainable development
+    "AR6-IMP-Neg": "#B8BDAA",  # Neg: emphasis on deployment of carbon dioxide removal
+    "AR6-IMP-GS": "#B5B7CA",  # GS: less rapid near-term mitigation followed by a gradual strengthening
+    # AR6 Illustrative Pathway (IP)
+    "AR6-IP-ModAct": "#DDB6AB",  # pathway with moderate climate action
+    "AR6-IP-CurPol": "#F9C8B7",  # pathway modelling current policies
 }