Write a program that takes R, G, and B values as command arguments and then displays that color some way. It doesn't matter how, as long as at least 100 pixels have the color. Shortest code wins.
xterm: 18 bytes
xterm -bg \#$1$2$3
Input the color as three 1- or 2-digit hex numbers, e.g., if this command is in the file "xterm.sh", then
./xterm.sh ff 00 00 ./xterm.sh f 0 0
opens a terminal window with a red background.
If you prefer, you can omit the blanks between the components:
The "-bg" option is not available in some old versions of xterm. It's missing from version 271 but present in versions 215, 251, 261, and 297.
read c;set $c;printf '\e]4;1;rgb:'"$1"'/'"$2"'/'"$3"'\e\\\e[31mc\e[m'
Partially stolen from here.
Just input your values in hex (e.g. FF 00 00).
Here you have to input the values yourself.
Edit: For 100 Pixels+ Just choose font-size infinite :D.