This site investigates the four-color problem:
https://mathshistory.st-andrews.ac.uk/HistTopics/The_four_colour_theorem/
A beautiful gallery of fractal designs are found on these sites:
http://fractals.hauner.cz/1
This site shows the colorful answer to Problems 24 and 25:
http://reglos.de/pentominos/
This site has many links:
https://mathshistory.st-andrews.ac.uk/HistTopics/fractals/
This site discusses the Mandelbrot set:
https://math.hws.edu/eck/js/mandelbrot/MB.html
This a site dealing with pentominos. This site shows the all pentomino solutions (you need to click on one of the numbers to see the particular solution). It also has links to other sites.
http://www.gottfriedville.net/puzzles/colorgame/solutions.htm