Question

I have an application that has about one hundred users. We have a calender view in which every user can have multiple entries. Every entry of the same user should have the same color, but different users should have different colors. At the moment we have manually defined about 25 colors in an array, and the color a user gets is determined by

$color = $colors[$userid % count($colors)];

We don’t want to manually add colors for every new user, and selecting a color for each user seems a bit tedious too.

Is there a simple function to map a number (the users id) to a color (preferably RGB)? The next question that rises is; Is there a way to limit the function to a certain types of colors (ie. only colors in which white letters are readable)?

Answer 1

I don’t have a solution, per se, but a suggestion.

First, since you want each user’s colors to be unique but random, you could get started with some kind of numerical hash function. That gets you a random number that’s most likely unique. if your hash function is decent and your users are in the hundreds that should suffice.

Now you need to turn that into colors, preferably on the dark end of the spectrum. If you go with RGB, You need three numbers. To keep them dark, lets say you keep the total or R, G and B under 200. That’s a guess, of course, but it should be pretty dark. Pick a random number [index] between 1 and 3 – that’s the R, G or B you’re going to start with. Get a number between 0 and 200, and set record it for color[index1]. Now pick one of the remaining indexes, 1-3. For this, pick a number between 0 and color[index1] that you already have. Set the remaining number to 200-color[index1]-color[index2]. Now you have a randomish RGB value that should contrast with white.

I realize I never got much use from that hash function up front – maybe ditch it and just use the userid as a random(seed).