(DISCLAIMER: once you've done all of these steps, you need to unsub to autoicons on the workshop otherwise there will be conflicts.)

to change the colors of the weapon select icons, you have to extract the mod with GMAD.exe

gmad.exe is located in \GarrysMod\bin
the addon itself is located either in \GarrysMod\garrysmod\addons
OR
\steamapps\workshop\content\4000 (look for a folder called 2495300496 in this location)
(personally, i wasn't able to find it in either location, so i just downloaded it manually through a steam workshop downloader website.)

once you've located the .GMA file for autoicons, extract it with gmad.exe.
to do this, just drag the .GMA file onto gmad.exe.
if you did it correctly, the extracted folder should look like this: \ADDON_NAME_HERE\lua\autorun\client\. you should also see autoicons.lua inside of the client folder.
(the reason why i put ADDON_NAME_HERE is because when you extract the .GMA, the folder's name may be different for you. just rename that folder to autoicons to avoid confusion.)

once you've extracted it, open all the folders inside to find autoicons.lua, then open it with a text editor.
if you're using notepad++, go to line 815
if you're using normal notepad, CTRL+F and type local weaponselectcolor

now change the numbers in brackets to whatever you like!
keep in mind though, instead of the values being 0 to 255, it is 0 to 1 (with 2 decimal places of precision).
for example, if you wanted pink icons, you would type (1, 0.75, 0.87)

to convert 255 RGB color values to decimal, divide whatever value you have by 255.
for example, if you want an orange color, like (255, 115, 0)
divide 255 by 255, = 1
divide 115 by 255, = 0.45
divide 0 by 255, = 0
giving you: (1, 0.45, 0)

once you've changed the color values to your liking, save it. then go back to the root folder (the addon name), then put it in \GarrysMod\garrysmod\addons. if you copied the folders correctly, the filepath should look like this:

\GarrysMod\garrysmod\addons\autoicons\lua\autorun\client\autoicons.lua

and now, it should be done!

(REMINDER: you should unsub from the workshop version once you've done this, it will conflict otherwise).