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You might want to choose a different colormap. The structures have different values (CMA Standard Labels) which you can see in the bottom-left of the mricron window.
CMA Labels
10 Left-Thalamus-Proper 40
11 Left-Caudate 30
12 Left-Putamen 40
13 Left-Pallidum 40
16 Brain-Stem /4th Ventricle 40
17 Left-Hippocampus 30
18 Left-Amygdala 50
26 Left-Accumbens-area 50
49 Right-Thalamus-Proper 40
50 Right-Caudate 30
51 Right-Putamen 40
52 Right-Pallidum 40
53 Right-Hippocampus 30
54 Right-Amygdala 50
58 Right-Accumbens-area 50