Follow three step behavior for setting clade_recency

Per rationale in issue 1131 (#1131), switch to following a three step look up for setting build-specific parameters of colors:clade_recency.

workflow/snakemake_rules/common.smk

@@ -169,10 +169,15 @@ def _get_metadata_by_wildcards(wildcards):
     return _get_metadata_by_build_name(wildcards.build_name)
 
 def _get_clade_recency_for_wildcards(wildcards):
+    # check if builds.yaml contains colors:{build_name}:clade_recency
     if wildcards.build_name in config["colors"] and 'clade_recency' in config["colors"][wildcards.build_name]:
         return config["colors"][wildcards.build_name]["clade_recency"]
+    # check if builds.yaml or parameters.yaml contains colors:clade_recency
+    elif "colors" in config and "clade_recency" in config["colors"]:
+        return config["colors"]["clade_recency"]
+    # else return sensible default value
     else:
-        return config["colors"]["default"]["clade_recency"]
+        return ""
 
 def _get_trait_columns_by_wildcards(wildcards):
     if wildcards.build_name in config["traits"]: