Malawa Shrimp losing color

Hey everyone,

I have had 3 Malawa Shrimp (originally 5, but 2 disappeared) in my small 4 gallon cube tank for about 3 weeks now, and I notice that they have a lot less color compared to when I first got them--or none at all, just translucent white & the tiny organs are usually brown. They were a slightly translucent dark blue or brownish when I got them. I'm not too worried about it, because they seem to be eating biofilm and moving around, but was wondering what may cause this? Here are some facts about the tank:

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-4 gallon glass cube, bought at thrift store
-Only other occupants are baby MTS and 1 Sparkling Gourami (I think the MTS's larger roommate died somewhere, I haven't seen him lately)
-Aquatop sponge filter only, with cheap topfin air pump
-1 6500k CFL and 1 generic LED fixture which came with the tank
-Pretty heavily planted, with lots of duckweed, some hornwort, guppy grass, water lettuce, and vallisneria is the only planted thing in there
-Don't change water much at all; it's been about 1 1/2 weeks since I last changed water, because I want to see how things do on their own with as little intervention as possible
-I also haven't fed anything to the tank for about a week, because the Gourami never eats anything I give him/her anyway, and seems to be a great micropredator, eating tiny things like planaria that have been showing up. The shrimp seem to just eat biofilm and other things growing in there
-I have been using a bit of Flourish when the guppy grass starts to brown a little and look like it needs food
-I can test the water parameters if that would be good info to have, but I think the PH is around 7.6 and there shouldn't be any ammonia or nitrites. I'm assuming nitrates are low with all the plants

The shrimp almost always hang out on the piece of manzanita wood I have in there, and seem to just eat the clear biofilm that always forms on the branches. Would this be a reason why they have lost nearly all of their color? I suppose if they are not eating anything with "color" in it, it makes sense that they would become less colorful.

tl;dr- Is it normal for Malawa Shrimp to lose their color when eating only biofilm? Or is it a sign of poor health?

Malawa shrimp, like all shrimp, color up with respect to their background and lighting.  From the photo, your tank is fairly dim with a light-colored substrate.  This is a fantastic recipe for dull-colored malawa.  

If, on the other hand, you had a brighter tank with a darker substrate, you'd see them color up better.

Also, I've found that they seem to color up best when there's a threat in the tank.  You have a threat in the tank, but apparently that's not enough.

As long as they're active and picking away they're likely healthy and fine.

Cool thanks pbmax, I have heard that (color in relation to the substrate & lighting) about other shrimp but just wanted to make sure that losing color wasn't a sign of them being in distress.