Download the shaders pack. The Hue shader is probably your best bet. I dunno how to make a windows slider, but you should be able to create your own in 2-3 lines of code using active objects and mouse controls.
You can make a slider control pretty easily. Gamester's suggestion is good, or there's also a Colorizer object with a Hue control that would work too. Changing the sprite's colour will be your next problem
Necro reply. It went well I ended up using color replace so that you can change every color on the sprite with an RGB slider. Pretty cool. I can't post what I came up with because I am using it for a game I am planning on releasing soon but here is a screenshot. There are still some bugs but it functions the way I intended.
Oh quick question. Do you guys think it would be too overwhelming to allow the player to change all the colors of your character? Or should I break up the colors into groups. For example Base color, Shade color and highlight color for the jacket would become one group called "Jacket" Instead of changing those individually you would change all 3 colors uniformly. What do you think?