So with your shader, I could go with this for the gba:
parameters = "display_gamma;target_gamma;overscan_percent_x;overscan_percent_y;sat;lum;contrast;r;g;b;rg;rb;gr;gb;br;bg;blr;blg;blb"
display_gamma = "1.400000"
target_gamma = "2.200000"
overscan_percent_x = "0.000000"
overscan_percent_y = "0.000000"
sat = "1.010000"
lum = "1.000000"
contrast = "1.000000"
r = "0.740000"
g = "0.680000"
b = "0.680000"
rg = "0.085000"
rb = "0.085000"
gr = "0.245000"
gb = "0.245000"
br = "0.000000"
bg = "0.245000"
blr = "0.000000"
blg = "0.000000"
blb = "0.000000"
This one is for nds:
parameters = "display_gamma;target_gamma;overscan_percent_x;overscan_percent_y;sat;lum;contrast;r;g;b;rg;rb;gr;gb;br;bg;blr;blg;blb"
display_gamma = "1.800000"
target_gamma = "2.200000"
overscan_percent_x = "0.000000"
overscan_percent_y = "0.000000"
sat = "1.040000"
lum = "1.000000"
contrast = "1.000000"
r = "0.750000"
g = "0.690000"
b = "0.800000"
rg = "0.060000"
rb = "0.060000"
gr = "0.220000"
gb = "0.100000"
br = "0.000000"
bg = "0.245000"
blr = "0.000000"
blg = "0.000000"
blb = "0.000000"
So far, this extended image adjustment shader is good. It can reduce the white overlap limit and does the gamma better than what other gamma shaders could do.
Edit: I got some news related to the real GBA hardware. If you put it at direct sunlight, it looks near to nds shader while being slightly darker, and took photos and videos of it. I have to put a pic of it. Everyone saying about the blue-purple and yellow-green on some shades is somewhat true when put on direct sunlight. Maybe games like castlevania looks better on sunlight. Itās just a non modded GBA screen.