If you're using the V5 default theme, or any 'modern' theme that uses the latest V5 meter theming capabilities, you have two options:
1
As I described in
this sticky, disable the V5 colouring and do it the way we used to in the old days, using the theme tweak/configuration window. This gives you far less control over the meters' coloring in their many states, and their appearance will always be a compromise.
2
Unpack the theme. Steal 'meter_strip_h.png' and 'meter_strip_v.png' images from another theme with meters that you like, or grab an image editor and recolour the blocks : select the block you want to change using the marquee tool or magic wand tool, fill with whatever colour you like, save the image, then go back to Reaper and refresh the theme. Done!
---The Details---
Using the modern method means the 8 blocks give you total control over all the colours the VUs might be in any condition, which is a huge improvement over the old pre-V5 method. You can stretch or resize those images as much as you like and Reaper will squeeze them in without complaint, so long as they are a multiple of 8 on the edge so one block doesn't bleed into another when Reaper chops up the image.
a. Unlit. The background of the strip.
b. Lit. Will be swiped over to indicate signal.
c. Clipped Unlit. The background of the strip, when the channel has clipped.
d. Clipped Lit. Will be swiped over to indicate signal, when the channel has clipped.
e. Armed Unlit. The background of the strip, when the channel is armed.
f. Armed Lit. Will be swiped over to indicate signal, when the channel is armed.
g. Armed & Clipped Unlit. The background of the strip, when the channel is armed and has clipped.
h. Armed & Clipped Lit. Will be swiped over to indicate signal, when the channel is armed and has clipped.