Grey shade GREY it RGB with components (GREY, GREY, GREY). Therefore the algorithm (formula) for three numbers (R, G, B) and GREY is important. Frequently use (processing of digitalimaging) formulas (http://neuroface.narod.ru/files/mahfoudh_autoref.pdf)
(1) grey=0.3*R+0.59*G+0.11*B
Or
(2) grey = (R+G+B)/3
(if R, G, B, GREY = [0,1])
Results are lower.
0-initial picture
1-calculation under the formula (1)
2-calculation under the formula (2)
(since I lazy my example uses interpolation from labview above )