There are people who paint on canvas all the time-- they are called
painters! You should check resources relative to doing paintings.
I suspect that it's possible that the moisture from the new paint might
affect the other side.
Making a backdrop isn't as easy as it seems, I've tried. But maybe you have
the talent to do it.
I think your first stop is a store that sells art supplies. You may need to
pretreat the canvas with some sort of sizing. I would suggest painting it a
solid color, then while still wet,  use an irregular shape sponge to apply
several lighter shades of the same color toward the center, giving you a hot
spot.
Good luck!

    In general you can do whatever you want, or you can paint on single or
both sides if you wish.  Some people use regular paint, some uses cheap air
brush type paint can be found at Hobby Lobby store this will make the
backdrop much lighter, you can fold without worrying the paint will crack
etc..  How to paint? you can GOOGLE for more detail.

    Or if you are very good with Photoshop or similar then you can paint
directly to Photoshop (I do quite often).

Use a good quality vinyl matt emulsion, thinned and applied sparingly
with a short-pile roller or stiff brush in two or three coats to get
good coverage.  If you apply it too thick (in either sense) it is more
likely to crack or peel.

HI all,
I have done the same, I used a water bace  paint in a darker shade
than the front and It worked fine.
I hung mine on my wifes close line and brushed away. let it dry and
ready to go.
have a nice day
Buckshot

If you can roll up the canvas with no cracking showing than it was
probably originally painted with acrylic or oil-acrylic (alkyd) artist's
paints. If you can't roll it up because the paint is too thick and/or
crazed, it may have been painted with oil paint.

Either way your safest bet for the other side is to use a high quality
acrylic gesso to prime the fabric and then pretty much any acrylic
artist's paint or latex interior house paint. You can buy gesso in
standard white or various tints if there is any bleed-through from the
original side. Gesso is an acrylic primer which contains very finely
ground calcium carbonate and titanium dioxide.

If you use acrylic artist's paint, Liquitex, Holbein, or Grumbacher are
quite adequate for colorfastness; Windsor and Newton is higher quality
and has a higher density of pigment, but costs significantly more than
the competition. Avoid fugitive colors like lavender, crimson, pink,
etc. as they will inevitably fade under direct sun, photo lights, or
strobes. Most artist's acrylics have a colorfast rating somewhere on the
tube. For what you are doing synthetic pigments are probably better than
mineral pigments or precipitated dyes.

You should also get some acrylic matte medium to cut the standard gloss
in artist's paints. Don't thin acrylic paints with too much water or the
pigments can precipitate out. The standard rule is to thin the paint
with 50% water and 50% matte medium. Conversely once you are done and it
is dry (acrylic dries very quickly) you can give it a coat of an acrylic
matte varnish for even reflectivity.

For brushes you can get an assortment of cheap nylon-bristle flats and
brights (like a flat brush but with a curved end) at any art supply
store. For effects like sponging, rag rolling, and marbling,
instructions abound but really you just need to look at an example and
practice away. There are also faux finish kits available for marble,
granite, and other "natural" looks.

Chances are very slim that you will damage the existing painted side
after 30 years. One major benefit of acrylic paint is that it remains
flexible indefinitely (the Sistine Chapel restoration was done with
acrylic paints because of their versatility, reversability, and low
V.O.C. content -- this was painted directly on top of Michelangelo's
original, delicate frescoes.)

Good luck with your project!

--k