Controlling RGB led strip each color with a switch
Yes you can connect a switch between each color and ground (assuming you have a common anode led strip). Most common switches at an auto parts store can do 10 Amps, more than you should need. At 1.5m, you are probably looking at less than an amp for each channel/color.

Of course, you will be limited to 8 color combinations.

You should not see any decrease in light intensity at 1.5 meters even with all 3 channels on, as long as your power supply can do 12V at how ever many amps are needed for your power strip.

Side note, the controllers for these can be very small and use radio or IR remotes. They should fit in the picture frame or can hang off the wire if you use an extension.