Fix colormaps' corresponding negative labels for NumPy v2 type casting

Change data type from unsigned to signed int to fix `OverflowError` when adding in negative int labels to a sequence of `uint` labels.

magmap/plot/colormaps.py

@@ -124,8 +124,10 @@ def __init__(
         if dup_for_neg and np.sum(labels_unique < 0) == 0:
             # for labels that are only >= 0, duplicate the pos portion
             # as neg so that images with or without negs use the same colors
+            lbls = np.array(labels_unique[labels_unique > 0][::-1])
             labels_unique = np.append(
-                -1 * labels_unique[labels_unique > 0][::-1], labels_unique)
+                -1 * lbls.astype(libmag.dtype_within_range(
+                    min(lbls), max(lbls), signed=True)), labels_unique)
         num_colors = len(labels_unique)
 
         labels_offset = 0