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I'm visualising color systems with TikZ to my students, and here I'm creating a 3D set of axes which represent red, green, blue and their combinations as shaded spheres.

For some reason, the result looks alright on the Blue axis, but doesn't work as expected on Green and Red axes as when their values are increased the spheres stay gray. Where's the problem?

I've taken care that everything is represented in code in the same order, RGB. Code first sets up some variables about axes (x, y, z), then step size/distance between spheres and then the max value for assumed 8-bit color, 255.

The point is that the nested for eaches go through all integer values between 0–255 for red, green and blue (all combinations), the second tikzmath creates scaled variables to be used with color information, and then with every pass a shaded sphere is drawn with the proper color and positioning on the axes.

documentclassmemoir
usepackagetikz
usetikzlibrarymath

begindocument

begintikzpicture[scale=0.03]
tikzmathx = 300; y = x; z=x; step=51;maxvalue=255; %optimal step size 51
draw[->, very thick, red] (0,0,0) -- (x,0,0) node[right]R;
draw[->, very thick, green] (0,0,0) -- (0,y,0) node[above]G;
draw[->, very thick, blue] (0,0,0) -- (0,0,z) node[below left]B;

foreach red in 0,step,...,maxvalue
 foreach green in 0,step,...,maxvalue
 foreach blue in 0,step,...,maxvalue

tikzmathreds=red/maxvalue*100;greens=green/maxvalue*100;blues=blue/maxvalue*100;

shadedraw[ball color=red!reds!green!greens!blue!blues!] (red,green,blue) circle [radius=10cm];
 
 


endtikzpicture

enddocument

I do not know precisely what went wrong but I guess that there is some sort of unitarity problem when the fractions do not add up to 100. Here is some other method that does what I think you want.

documentclassmemoir
usepackagetikz
usetikzlibrarymath

begindocument

begintikzpicture[scale=0.03]
tikzmathx = 300; y = x; z=x; step=51;maxvalue=255; %optimal step size 51
draw[->, very thick, red] (0,0,0) -- (x,0,0) node[right]R;
draw[->, very thick, green] (0,0,0) -- (0,y,0) node[above]G;
draw[->, very thick, blue] (0,0,0) -- (0,0,z) node[below left]B;

foreach red in 0,step,...,maxvalue
 foreach green in 0,step,...,maxvalue
 foreach blue in 0,step,...,maxvalue

tikzmathreds=(red/maxvalue)*255;
greens=(green/maxvalue)*255;blues=(blue/maxvalue)*255;
definecolormycolorRGBreds,greens,blues

shadedraw[ball color=mycolor] (red,green,blue) circle [radius=10cm];
 
 


endtikzpicture
enddocument

EDIT: Appended a version with unique color names. With all these different viewers around one never knows. The output is the same.

documentclassmemoir
usepackagetikz
usetikzlibrarymath

begindocument

begintikzpicture[scale=0.03]
tikzmathx = 300; y = x; z=x; step=51;maxvalue=255; %optimal step size 51
draw[->, very thick, red] (0,0,0) -- (x,0,0) node[right]R;
draw[->, very thick, green] (0,0,0) -- (0,y,0) node[above]G;
draw[->, very thick, blue] (0,0,0) -- (0,0,z) node[below left]B;

foreach red in 0,step,...,maxvalue
 foreach green in 0,step,...,maxvalue
 foreach blue in 0,step,...,maxvalue

tikzmathreds=(red/maxvalue)*255;
greens=(green/maxvalue)*255;blues=(blue/maxvalue)*255;
definecolormycolor-reds-greens-bluesRGBreds,greens,blues

shadedraw[ball color=mycolor-reds-greens-blues] (red,green,blue) circle [radius=10cm];
 
 


endtikzpicture
enddocument