The rapid increase of the amount of impervious surfaces in urban areas has generated heat islands and other adverse environmental impacts that subsequently increase greenhouse gas (GHG) emissions. Local governments are now looking for an index to encourage building owners to increase green areas, including green roofs and green walls. Currently, several building green value (BGV) indices can be used to assess building greenness, including the green coverage ratio (GCR), biotope area factor (BAF), green factor (GF), and green plot ratio (GnPR). However, these indices use different sub-indexes and weights, such that the values they output vary significantly. To analyze their applicability, this study compares output values of the BGV indices and estimated GHG emission reductions for three typical buildings under three conditions: no green roof or green wall; a green roof; and a green roof and green walls. The GCR does not include green walls and thus cannot reflect reductions in GHG emissions from green walls. The BAF values are highly correlated to associated GHG emission reductions. The trend of GF values vs. associated GHG emission reductions is less consistent than that using the BAF method because the weights of GF are not consistent with associated thermal resistance improvements. The correlation between GnPR values and associated GHG emission reductions is the strongest among all BGV indices because the ratio of leaf area indexes for green roofs to that of green walls is close to the ratio of U-value reductions from green roofs and green walls.