Is it harder to see a Gelatinous Cube with darkvision?

Question

The description of a Gelatinous Cube reads:

Transparent: Even when the cube is in plain sight, it takes a successful DC 15 Wisdom (Perception) check to spot a cube that has neither moved nor attacked. A creature that tries to enter the cube's space while unaware of the cube is surprised by the cube.

With Darkvision you can't discern colours and only shades of grey, although it does say you see as if you are in dim light.

My question therefore is:

Is it more difficult to see Gelatinous Cubes when there is no light and characters are seeing the world with their Darkvision?

Answer 1

The cube will be lightly obscured for characters with darkvision and no light sources.

Looking at the rules for Vision and Light:

In darkness without a light source, a character without darkvision can't see the cube:

A heavily obscured area—such as darkness, opaque fog, or dense foliage—blocks vision entirely. A creature effectively suffers from the blinded condition when trying to see something in that area.

In darkness without a light source, a character with darkvision sees as if in dim light, meaning they have disadvantage on sight-based perception checks:

Within a specified range, a creature with darkvision can see in dim light as if it were bright light and in darkness as if it were dim light, so areas of darkness are only lightly obscured as far as that creature is concerned.

...

In a lightly obscured area, such as dim light, patchy fog, or moderate foliage, creatures have disadvantage on Wisdom (Perception) checks that rely on sight.

In darkness with a light source, a character conducts perception checks to see the cube normally if it is in the area of bright light cast by the light source.

Bright light lets most creatures see normally. Even gloomy days provide bright light, as do torches, lanterns, fires, and other sources of illumination within a specific radius.

Nothing in the Cube's Transparent trait indicates that darkvision provides any special disadvantage in spotting the Cube. Relative to a group of characters with light sources, characters with darkvision will have a harder time seeing the Cube, but they will have a harder time seeing anything else as well.

Answer 2

Rules wise it's pretty clear cut. If the darkvision allows you to see "As if in bright light" then it doesn't matter for the cube. However you can easily justify a +2 circumstance modifier for the difficulty, think of it as someone in a green outfit posing in front of green foliage - there is nothing in the spotting rules about your outfit matching the scene, but as a GM you can justify it.

Another fringe case might be the character moving very fast. Dark Vision typically has a range of 60 feet. If you move at more than 60 feet per round the GM might rule that the check is made more difficult since you have very little time to spot the jelly.