Gimp

Remove color

• pick color with with the Color Picker ( key o)
• select fuzzy select (key u ) set threshold as 0
• click on the color to select
• press ctrl+x to remove

Replace color in image

• select original color with the Color Picker ( key o)
• select menu Colors, Map, Color Exchange
• Paste the RGB code HTML notation in the from color
• Write the target color RGB code HTML notation in the to color
• click OK