I think you just need to change it from single-coloured disc to "bitmap disc", which is the NewDark particle type that allows you to use the colourpicker on its colours.
Quote:
The particle type "Bitmap disk" has been added. It works like a "Single-colored disk" with enhancements, but
instead of always using an internal disk image, it can use a user specified bitmap, like "Scaled bitmap" particles
also do. If bitmap name is left empty then it'll use the same texture as "Single-colored disk" particles, this
makes it easy to convert "Single-colored disk" particles to "Bitmap disk", to benefity from improvements like for
example lighting support or proper screen edge clipping. Unlike "Scale bitmap" particles, if the bitmap is animated
and animation is enabled, each particle will play the animation. Another difference to other particle types is that
the "Transparency (alpha)" property (and "Distance Alpha") affects them. The following "Bitmap disk" specific
parameters are available:
"bm-disk flags"
---------------
"Use Lighting" - Apply object lighting to particle group (only a single lighting value at the
origin is calculated so it may not be suitable for large particle groups).
"Fade-in at Birth" - Fade in alpha from 0 to "alpha" during birth phase.
"Grow-in at Birth" - Grow particle from (near) 0 size to "size of particle" during birth phase.
"Random Start Frame" - Randomize start frame per-particle for animated bitmaps, if animation is disabled
this will select a random frame for each particle.
"Play Ani Once" - Play an animated bitmap once and stop on the last frame.
"Use RGB Colors" - Use alternative RGB color param(s) instead of the normal palette based color(s)
"Fade RGB->2nd" - Enable color fading from primary RGB to 2nd (requires "Use RGB Colors")
"Fade RGB->2nd->3rd" - Enable color fading from primary RGB to 2nd to 3rd (requires "Use RGB Colors")
"bm-disk birth time"
--------------------
Duration of birth phase, if 0 there is no birth phase, make sure that birth time isn't too long so it
interferes with "particle fade time".
"bm-disk rot (vel, offs, rand)"
-------------------------------
Rotate particle texture. It doesn't rotate the actual particle, only the UV coordinates, so the bitmap's
contents should fit a disc-shaped area to avoid the poly edges from becoming apparent. There are three
rotation parameters, 'vel' is the rotation speed, 'offs' is how much each particle's start rotation is
offset from the previously launched particle, 'rand' is the amount of random rot speed variation to apply.
"bm-disk ani frame time"
------------------------
Controls the rate at which animated bitmaps animate, if the value is 0 then animation is disabled.
Be aware of that using animated textures may degrade performance because rendering of the particles
can be split up a lot when switching textures back and forth, so it may be a good idea to not have
an excessive amount of particles in such cases, or only use the effect in areas that aren't performance
heavy.
"bm-disk grow speed"
--------------------
The speed at which particles grow after the birth phase, defined as percentage of "size of particle".
For easier selection of palette index for particle colors there's a "pcolor_picker" command, which opens a color
picker dialog that will output a palette index with a color that is the closest match for the picked color.
